This is a known issue. The release notes for 10.6.2 suggest the following work around.
https://docs.mendix.com/releasenotes/studio-pro/10.6/#known-issues
Hi Natalie,
In you document template under custom styles, add this font and try.
font-family: "centurygothic";
If this font doesn't work then you need to look for a font that supports the characters that you are trying to insert.
Hee Natalie,
Check out this post, i think it is the same problem:
https://forum.mendix.com/link/space/microflows/questions/130627
Can you also file a ticket at mendix the more people file a ticket the more attention it gets from Mendix