Vivaldi remembers the setting for the last item it processed. So if you play an MQA encoded track and switch off afterwards, when you reboot Vivaldi will be configured to play another MQA track.
If instead of MQA you select , say, a redbook 16/44.1 recording then Vivaldi will remember what your preferred settings for that format are and switch to them.
As for missing a few seconds of music when changing from MQA to ordinary PCM, there have been a few issues raised on this forum about the way MQA works in the dCS implementation and it would be interesting if Andrew is able to confirm whether this is or is not another one that is known.
I currently have the same dCS setup as you but cannot replicate your issue as I am not a Tidal subscriber. Of course Qobuz , which I do subscribe to, streams hi-res flacs and the problem does not arise. I would, however mention that Qobuz allows users to set a buffer time before playback no doubt anticipating broadly similar issues in certain circumstances. However that relates to their own player applications and I presume would have no bearing on Mosaic.