Launch issues and fighting games go hand-in-hand. UNI2 just a day prior was suffering from crashes when players tried to get into netplay, and now Tekken 8 is suffering from the same fate, albeit not as consistently.
The game peaked on Steam with 49k concurrent players, absolutely fantastic number for a fighting game, only 20k below Street Fighter 6 and over 30k above previous Tekken.
Combined with the console population, it's not hard to see how this could cause some strain on the servers, leading to plenty of crashes and a mixed review rating for an otherwise exceptional game. Worth noting here that these issues weren't present during the tests or previews, so it's it might just be a typical launch issue that might be hard to predict or account for on developer's end.
Don't worry though, Harada and Tekken developers are aware of the issue, and we should see a fix soon enough.