I could reproduce your error on my end, and it really seems a Bug.
What I did to fix it was closing the project, changing my license to Studio (was using Studio Pro) and then changing it again to Studio. After that, opening the same workflow and sending AltGr+2 on my keyboard, typed correctly the “@” symbol.
→ My steps were a bit random, restarting UiPath seems to solve it, temporarily.
Also a check you can do: if you type Chr(64) on the immediate pane, does it correctly write “@”?
Chr(64) indeed works, as does copy-pasting,
I’m pretty sure it worked at some point, but I think since we last updated it still an issue. I’m on 21.4.4, and as we haven’t updated to 21.10 yet, I am interested to know if it is fixed in the latest version.