So, after spending an hour googling through different people having language / character issues with alt+gr on windows in Illustrator, I think I have identified what is happening (see screenshot)

Win10 can do mixed display language / keyboard layout combinations that most Adobe products will handle just fine, but Illustrator seems to set keyboard language based on general language set, not on keyboard language set.

So, if I type my normal alt gr+q for @ symbol or alt gr+e for € symbol on a german layout while using english language with german keyboard, it does not pass through as altgr+e but instead interprets it as alt+ctrl+e (export for screens) even when I'm inside the text tool.

When I use german language and keyboard instead of english with german keyboard, it correctly uses the symbols on alt gr and "export for screens" on alt+ctrl+e.

This has apparently been an issue since 2017, but only affects people using mixed settings with keyboard layout different from general language.

Quick fix is making a language/kb profile in win10 and using that for illustrator, then switch back for indesign photoshop or whatever else you use. It's dirty but it's a better workaround then copy+pasting every fifth character when writing non english text.