@Biche
Hm, we are at Vivaldi 6.7.3335.68, 124 is the Chromium number, anyway.
I sync about 140 notes between different devices and we have no other reports about this.
As you are starting with Vivaldi on Android i would reinstall it and start from scratch.
It can take up to one hour to sync if you not trigger it manually.
All is synced, it can take a while and I would wait for some time.
Maybe you mess up something trigger it manually during the first sync process.
Cheers, mib