Figma itself never inserts anything. Perhaps you are pressing some kind of shortcut that intercepts other software and inserts characters. Try closing other programs or changing their shortcuts that they intercept.
Nowadays this forum in the Figma menu is named «Support forum», so it got me confused, because I was expecting a bug report form with an answer from the team, not this. I guess your message is also a support in a way, because I’ve found that form and reported to the team after all