I knew I had seen this before, but it doesn't look like it was ever resolved, and is the only other instance I'm aware of:
https://www.project1999.com/forums/s...d.php?t=379235
Have you tried changing fonts? Is it only in /tells? Do you see it replicated in your logfile? Is it just the one friend? Could they possibly have your chat going to a different window / chatbox that isn't quite the right size?
Grasping at straws here... - could they just be messing with you?
Edit: - oh yeah, you do reboot on occasion, right? Does the "missing character" issue also occur when mouseover text isn't working?
-Mcoy