I have a s20 5g and tried the suggested fix re. tuning off the "show in front" option. It didnt work ! Not only that I have noticed that if I dont send my message quickly enough when using the Messenger App the text get cleared, probably at the same time as the messaging app get started automatically. A fix is definitely needed some where here.
I also have an S20 5G. I don't think it's malware. I have AVG installed and scanned regularly. The always on top setting = off has fixed it for me.
And same here. Galaxy S20 FE 5G. It's annoying, but I would be content to learn that it is not someone making screen shots and messaging them to themselves.