This problem is worse: Blender crashed using Shift-H and Alt- H in sculpt mode. This is the latest build created Today, 5:34 AM: The UN-hidden didn't appear initially until playing with Dyntopo again and then trying to get the rest of the mesh back with Alt-H caused the crash:
Process: Blender [16535]
Path: /Users/USER/*/Blender 2.91a2.app/Contents/MacOS/Blender
Identifier: org.blenderfoundation.blender
Version: 2.91.0 (2.91.0 2020-09-22)
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Blender [16535]
User ID: 501

Date/Time: 2020-09-22 12:40:25.305 -0700
OS Version: Mac OS X 10.15.6 (19G2021)
Report Version: 12
Anonymous UUID: E5A6D88C-4F8E-4F17-D3EF-A97E347D933F

Sleep/Wake UUID: EA3C6A7B-FE6A-46AA-99F7-C0A892FA1093

Time Awake Since Boot: 470000 seconds
Time Since Wake: 2200 seconds

System Integrity Protection: enabled

Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY

Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler [16535]

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_malloc.dylib 0x00007fff6b94df3e nanov2_allocate_from_block.cold.1 + 31
1 libsystem_malloc.dylib 0x00007fff6b92e9b1 nanov2_allocate_from_block + 574
2 libsystem_malloc.dylib 0x00007fff6b92df2b nanov2_allocate + 130
3 libsystem_malloc.dylib 0x00007fff6b92de58 nanov2_malloc + 56
4 libsystem_malloc.dylib 0x00007fff6b92dd7a malloc_zone_malloc + 104
5 libsystem_malloc.dylib 0x00007fff6b92dcf5 malloc + 21 .....
ad nauseam.....

I suspect that my GPU is not a good candidate for using Blender. I seem to find a lot of problems that no one else has mentioned yet.

There needs to be a way of locking the GPU out of using this APP and using just the CPU apparantly--maybe there is and I just don't know about it. I can then go through some of the problems I have found and see if they just disappear.... Let me know.

My description above is exactly how the Hide feature is not working and I have repeated it multiple;e times with the same result, even yesterday. It is really obvious if it doesn't work when going from Modeling with just the horse head showing (still all selected or not) and then to Sculpt and the whole horse is there again until the Dyntopo / Smooth Shading is selected and just the head is there again.

Here are requirements for Blender: https://www.blender.org/download/requirements/
What is your GPU? Can you click on Help > Report a Bug and update description?

There are reports with GPU glitches and such on MacOS, since they decided to drop support for openGL. Using CPU only is technically possible with SW openGL emulation, but it is very slow.

My GPU is a premium buy for Mac:

Radeon Pro Vega 48:

Chipset Model:	Radeon Pro Vega 48
Type:	GPU
Bus:	PCIe
PCIe Lane Width:	x16
VRAM (Total):	8 GB
Vendor:	AMD (0x1002)
Device ID:	0x6869
Revision ID:	0x0000
ROM Revision:	113-D0650E-072
VBIOS Version:	113-D05001A1XG-011
Option ROM Version:	113-D05001A1XG-011
EFI Driver Version:	01.01.072
Metal:	Supported, feature set macOS GPUFamily2 v1
Displays:

iMac:

Display Type:	Built-In Retina LCD
Resolution:	5120 x 2880 Retina
Framebuffer Depth:	30-Bit Color (ARGB2101010)
Main Display:	Yes
Mirror:	Off
Online:	Yes
Automatically Adjust Brightness:	No
Connection Type:	Internal

The rest of the system is 1T SSD drive with 500G SSD USB 3.0 for cache / extras.... The system was bought the end of 2019 with the hope of doing this kind of work since my 2009 Mac would nearly catch fire doing a cycles render. I have been having continuous problems in unexpected ways and wonder if it is even worth reporting here anymore as it obviously is not a supported system as no one else has these problems. Very disappointing indeed. I am not going back to MS after the constant crash problems with Vista. I wonder if any video card available for this system would work any differently with openGL not supported.....

USING: Blender-2.92.0-5424b4821d28-macOS (6 Jan 2021)

Subdivided default cube 3 times and clicked on Modeling tab. Selected a corner section and hit Shift-H. With selected corner section only showing, clicked on Sculpt tab. Clicked on Dyntopo square and changed Resolution to 11.7 and did some quick sculpting.

Clicked Opt- H to see whole object in Sculpt and nothing happened.  Clicked Modeling Tab and Opt- H and brought back whole mesh.  Selected same approximate area again and Shift-H and back to Sculpt with section.  Dyntopo is no longer selected.  Each time I go in and out of Sculpt I need to turn Dyntopo on again.  This is unlike the other settings and going back and forth to make modifications and having to turn this on every time is confusing and costly if you forget to turn it on and try sculpting without it.  I don’t see why Dyntopo is not on by default for Sculpt instead of being off.

 Opt-H brings the rest of the mesh back into view in Sculpt and going back to Modeling still has the selection available to repeat if desired EXCEPT when Dyntopo is clicked on.  Then Opt-H has no effect.  The only way to see the rest of the mesh is to go back to Modeling and click OPT-H or go to Layout.

 Too bad the view isn’t the same when changing from Modeling to Sculpting at least initially for working back and forth.˙