Having the same issue for months.
I have 2 PC-s connected to my android phone. It ALWAYS disconnects from either one from the PC's including the data sync and flow as well. There is no warning, no information, nothing, at the time when I understand what is going on, I have lost dozens of bookmarks and it is really really frustrating.
I have wrote emails to opera support, i have reported bugs and I have wrote this in 'suggestions how to improve" yet no answer and no fix. I have this issue at least 8 months.
Additionally I have reset everything and reconnect successfully just about 2 weeks ago and today i got the same problem again. I don't think that anything works and I think it's their internal issue.
Probably I`ll switch to brave browser after using opera for 5 years.
Edit: Possible fix is this but don't expect to be a long term solution, usually it disconnects very often.
Go to opera.com/connect -> scan the QR code with your phone -> confirm numbers.