System Information
Windows 10 Home
NVIDIA GeForce GTX 660

Blender Version
Broken: 2.80, 87aca8bd02bc, today's build (2019-03-14)
Worked: 2.79b release

Short description of error
It seems that you can get the correct vertex locations of a tapered curve object only once per session. After that, if any changes are made to the taper object, the vertex locations are not updated accordingly.

Only during the next session (after saving and reloading the file) are the vertex locations updated, but again, only once for that particular session.

Exact steps for others to reproduce the error
The issue came up while working with Animation Nodes (2.1.4) and I think the easiest and clearest way to explain the issue is to just show it in that context.

Although the issue came up in Animation Nodes, it turned out to be an internal problem in Blender. If necessary for anyone, here is another video displaying the issue in Blender without Animation nodes...

... and here are the steps to reproduce the error as seen in the second video:

1. add a bezier curve (Curve A)
2. increase Geometry > Bevel > Depth to 1
3. add another bezier curve and assign it to Curve A as its taper object
4. duplicate Curve A (with Shift D)
5. move the copy to another location so it is easier to see what is happening
6. select the taper object, go into Edit Mode and change the control points
7. observe that both Curve A and its copy have changed their shape
8. convert the copy to a mesh object (Object > Convert to > Mesh from Curve)
9. observe that nothing strange is happening, the copied curve objects shape doesn't visually change, it just is converted to a mesh object
10. now repeat steps 4 - 8
11. observe that the shape of the second copy has changed to the same shape as the first copy, expected behavior is that the second copy doesn't change its shape at all when it is converted to a mesh

Simple .blend files
File used for the first video:

File for second video: