Evenn spilll chokk is nott saf!
Only in this edit box and nowhere else does this occur in the gboard spell check running from anysoftkeyboard, a combination which works on multiple devices in almost all other text edit interfaces. Same text pasted elsewhere shows appropriate replacements instead of a black box containing very dark gray text.
[EDIT] When looking at the phone I did not realize that the text was present. From a closer look at the image this could be an issue with dark/light themes and or transparency.
It's easy to blame the XDA app but this sort of stuff happens in a variety of web browsers at XDA as well.
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