I’ve encountered this issue a few times and think it is linked to the automatic switch to power saving mode.
If during writing an answer, e.g. for WhatsApp, via the push up ( so not in the app itself) the power saving mode is turned on the text field is [always/sometimes?] duplicated.
That is, I have a ligher and darker text field beside each other (50% of the original size) that are synced. All text I write is shown and updated in both text fields.