I can confirm that this is the case using Pop-up menus and is does seem like a bug.
Drop-down List however, works fine. Maybe you can use that?
The other issue is harder for me to reproduce as I don't immediately know how to insert those characters via the keyboard.
Thanks for confirming that it's a FM problem and not my computer, Andreas.
You're right, the drop-down list works fine and is an acceptable alternative, although in this case I would prefer the pop-up menu.
The other combining unicode characters can be entered on a Mac using the character pallate. I'm not sure how to enter them on a PC.
This is definitely a FileMaker Pro problem. Development is aware of this issue. As you have discovered, this only happens with pop-up menu. No additional information is available at this time.
I tried inserting the combining characters using the character pallette on the Mac and you are right that they do not insert when you double click the character as per usual.
I was able to insert them by dragging them onto the text field, I don't know if that works for you.