Thank you Marco (@marpet) and George (@gnwiii) , now it is much more clear to me.
A follow up question is how to properly reproject to UTM. I may start a new thread if it seems more appropriate. To begin with, I found a lot of questions about OLCI reprojection and problems with per-pixel geocoding mode, but I did not find my case there.
The case is that the image gets truncated after reprojection to UTM 31N zone.
I presume the image gets truncated.
I was able to reproject, copying crs from the product I had (“What I want”, which is UTM 31N as well), but the desired product will not be with me all the time. Setting the desired file resolution, width and height did not help.
Is it a known issue of truncating?
Shall I parametrize the Reprojection operator differently? (Graph processing command does not change with crs specified as EPSG 32631 or copied from the product).