Tldr Using AltGr + A causes (in the following order) Ctrl + A and then Alt + A inputs to be processed but it should only result in Alt + A .
I’ve noticed that whenever I try to type ą character (Alt + a on Polish keyboard) in chat (later I’ve reproduced that in a TextBox - at first I thought it’s chat-related issue), the whole content of the TextBox is being cleared.
I use Windows 11 system.
Video which shows the issue: https://youtu.be/EB-f4d1hfTo
The TextBox content is only cheared when ą is typed for the first time. If you keep the Alt + A pressed, more ą characters are being typed correctly without clearing the TextBox’s content. The content is only cheared once you release and press that combination again.
I’ve asked my friend to reproduce the issue but he could not. I’ve looked up the issue and it appears to be caused by the same thing as described here:
tl;dr: Right Alt is interpreted as Ctrl + Alt, so Alt + A becomes Ctrl + Alt + A. Roblox first handles Ctrl + A (select all text) and then Alt + A (type ą) so as the result the whole text is replaced with ą.
I’ve opened an online keyboard tester and everything seems to match - whenever I press right alt (the that is normally used to type ą when typing using 10 fingers) it’s being interpreted as Ctrl + Alt and on my friend’s device it is just Alt w/o the Ctrl.
Not sure what exactly causes the difference in behaviour but for sure Roblox should be able to handle that case correctly instead of just clearing the whole TextBox.
Update: Welp. I’ve just looked at my keyboard and noticed that the right alt is even labeled as AltGr so I might have found the cause of the difference. 
Expected behavior
The ą character should be correctly typed without clearing the TextBox’s content.