I was about to give up on this, but I found a better solution.

@MarnixCroes @Kruwed could you please test if this fixes the issue on your OS as well? Ty.

This issue bubbles down to Avalonia's AutoCompleteBox or the way we use it (couldn't isolate one explanation)

AutoCompleteBox.SearchText property is not correctly updated when the Suggestions array contains the new text to be written.
So when I write o then f, AutoCompleteBox.SearchText will be equal to of.
But when I add another f, AutoCompleteBox.SearchText will still be equal to of, as Suggestions contains off

Because of this, trying to add subsequent letters doesn't work because we force the Text to always be part of Suggestions, but the text we try to write is ofi according to the component, which therefore doesn't allow it.

221a4ca is extremely dirty but it more or less works -> It uses reflection to check which part of the AutoCompleteBox is actually a text and which is the suggested part. If there is no suggested part, it overrides typedFullText with the full text, in my example offi