Thank you for your post, and I apologize for the late reply.
Before I report this issue, I need you to update to FileMaker Pro 11.0v3 and verify the problem still exists. If so, I would like a sample file along with exact steps to reproduce the problem. Please check your Inbox at the top of this page for instructions where to send the file.
Yes, verifying that this issue exists in FileMaker Pro 11.0v3.
To reproduce the problem no special files needed, simply do the following:
(1) create a new (Unicode) text field in any table;
(2) set formatting in layout to use a font that has glyphs for top plane PUA code points (U+100000...);
(3) import UTF-8 text with those code points into that field;
(4) see that text displays correctly (or not);
(5) try pasting one character with such top plane code point into the field;
(6) see that text displays incorrectly (not associated with PUA font);
(7) try un-do and/or alternately cutting and re-pasting;
(8) eventually text might display correctly.