Obviously I can't see your devices or settings @Easty but when I create an event in my Samsung Calendar on my N10+ 5G or N20 Ultra it automatically updates on both phone's and on my Samsung Tablet too.
The phone's are on the same software version and the tablet is a 2011 model so is on it's most newest available.
I can only suggest to turn Off Sync and then re enable it and then see what happens.
As for One Drive I'm still using Samsung Cloud until I'm forced onto One Drive.
If I'm honest I'd rather stay with Samsung Cloud as it's worked flawlessly for me.
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
I am havin the same issue and also suspect the half@$$ integration job they did with MSFT OneNote to be part of the problem. I just bought the Tab S6 Lite for work notes specifically because it synced to my Note10 and existing notes... My new pages aren't being synced to my Notes app on my phone now and I've only found a few threads about it on these community pages, no suggestions or feedback on a fix so far. Another potential issue I thought could be the problem is that supposedly verizon phones don't support samsung cloud, however, I don't believe I had this problem until recently.