Its not overheating, but it heats up quicker than the other engine. It seemed to overheat, but it was faulty sensor. But the quick heating is still strange.
Anyway, oil is clear. I actually started to suspect the thermostat myself. Its new, but the housing is quite corroded.

There is one more strange symptom, the engines sometimes flood when idling in gear after a hard drive. This is as strange as it seems, caused by excessive blowby. When crankcase vents are disconnected then it does not happen.

The engines are rebuilt, new piston rings etc. So... maybe I am a little overcautious when believing that there is no issue, because I have had a horrible amount of issues. Most of them solved, but troubleshooting and fixing sloppy rebuilding has taken two years and the engines have come out repeatedly (oil pumps, camshafts, you name it).

Now the rest works - but these bubbles are a bit anomalous... so I´m cautious. Seems unrealistic that now it works ok

Both engines have the bubbles, but only one heats up quicker? Perhaps just a slower thermostat?

It is possible that the one that heats up slower, is leaking from side or a bit open.