System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 456.38

Blender Version
Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash: rB0f45cab862b8
Worked: version: 2.90.1, branch: master, commit date: 2020-09-23 06:43, hash: rB3e85bb34d0d7

Short description of error

If a numeric figure is entered after pressing "R" to rotate an object by a set amount in object mode or if bpy.ops.transform.rotate is called via a script or the console, objects rotate in the opposite direction to intended.

A similar bug was present in 2.80 https://developer.blender.org/T66527 but this had a workaround that if you set the orient matrix in a script it would force the rotation to be in the correct direction. This workaround no longer works.

This bug also effects mesh edit mode and pose mode!!!

Exact steps for others to reproduce the error

Launch startup file.
Go into top view
Go into orthographic mode.
Select default cube
Press "R" on the keyboard and enter 15

Result:
Cube rotates -15 degrees rather than +15 degrees.

Alternatively:

Launch startup file.
Go into top view
Go into orthographic mode.
Select default cube

then in the console:

bpy.ops.transform.rotate(value=0.261799)

Result:
Cube rotates -15 degrees rather than +15 degrees.