Hi Brian
The key thing that effects this isn't what you've done in GSC, it's what you've done in terms of redirection (301s) on the site(s) themselves.
Don't forget no one has to have GSC and it's primary purpose is simply to report information to you from the sites. The grunt work of any site move is done via crawling. That's the primary reason Google checks if the 301s exist and are correct (as part of you telling it there is a move).
When you do a site move, you are essentially, certainly initially, starting from ground zero.
Google won't take your word that the content has just moved, it'll check and re-index everything (once it follows the appropriate 301). If you think about it logically, it has to do that, otherwise you could have a site about 'fluffy dogs' and 301 it (after it was ranking well) to hardcore porn. This can and pretty much will mean that stuff it was just scanning quickly and was to all intents happy with previously, gets a much more detailed appraisal.
During the process you'll very likely get extreme instability. Checking the content is a multi-pass, multi server process (it's not just one bot doing everything). As such you'll see pages indexed at both locations together (or not at all) with large variance in ranking etc, all as different parts of the algorithm are assessed and re-applied.
It typically takes 6 months minimum to settle and it's certainly not a process for the fainthearted. The more you play around with the process, the longer Google will take to figure out what is what.
Finally, if you don't leave the 301s in place for the long-term, you'll also lose any 3rd party links that went to the old site(s) too of course. Nothing is moved without the 301s.