Steps to reproduce this issue:
Launch demo modal
Popover gets stuck, doesn't "follow" the button
What should happen:
Click to toggle popover
Hope it helps :)
I've just found another issue that may be related: if you mouse over the Popover and try to scroll, it will scroll the page containing the modal, not the modal itself.
It's to do with the closest containing element (E.g. with relative positioning, etc). Easiest fix would be to add position: relative to the .modal-body. This will fix both the issues you pointed out as well.
Add relative positioning to .modal-body - Fixes #5991
This one seems a duplicate of #4605, just for reference.
(less) Add relative positioning to .modal-body - Fixes #5991