One thing to remember is that an issue like the one described here is very difficult to analyze, because it takes so long for the problem to develop (E.g. I may be seeing the same/similar issue, but it only becomes noticeable after several days, maybe over a week of running the same instance of Vivaldi)
Running in a debug environment for that kind of time period is not really practical, unfortunately. Unless we have an inkling of where the issue is (and can instrument it), we essentially need steps that can reproduce the issue relatively quickly (which often means much less than an hour), and I suspect that is not really practical in this case, either.
Another thing to keep in mind is that details such as what features you are using, e.g. Mail, extensions, other configuration choices, other usage factors (e.g. tab usage patterns), may play a part. In my quick skim just now, I noticed a number of posts that did not seem to include such information (in some cases there may be information, but hidden inside a lot of other text. Pro Tip: Using short bullet point descriptions to list information is very useful).
Knowing that kind of information may help discover what is, and (in particular) what isn't, involved in creating the problem, which may help narrow down where to look for the problem. (E.g. I have Mail enabled, no extensions, only minor configuration changes, and opens some multi-bookmark entries at least once a day, which is usually how I notice the problem, I always close tabs after reading, which creates a long history of closed tabs, which may conceivably be related to the cause of the problem.)
Regarding opening in other profiles and Private Window not being affected, the reason and important part there is that those are running in completely different, separate processes than the main profile and window(s), so they are isolated from whatever is affecting the main profile.
BTW, @RasheedHolland, you may want to check back on who you have been replying to in this thread; you were actually replied to by, and replying to, a Vivaldi dev over a month ago.