Since you have requested data on this problem, I will add mine here, since I had this problem with Mint 20.0 and have reproduced it with Mint 20.1.
Using Linux Mint 20.1, Ulyssa, clean install about a month ago.
I had this problem with Vivaldi on Mint 20.0 (upgrade from 19.3) and lack of passwords and logins in Vivaldi was one of the reasons I did a clean install. At first, no problem. Vivaldi was once again storing my passwords and entering them for logins. Hooray!
Then suddenly about a week ago they all just disappeared. Vivaldi still asks me when I log in if I want to save the passwords and login info (unlike when I was using Mint 20.0), but when I check my "saved" passwords and logins, Vivaldi tells me I don't have any.
Extensions used: Joplin Web Clipper, LINE app, NordVPN, Jitsi Meetings, social fixer for fb, XDM download manager, and TabCopy.