Why ETA for EU settled in UK?

I am flying next week to Málaga for a short break and I checked in for both flights, return one from Málaga won't show because they need to verify my documents, I already stated I don't need ETA, it's absolutely shambolic.

I even did ETA for some stupid reason and of course there is no UK as a country listed there.

I'm holding a Polish passport btw but pre settled over 15 years.