They Might Up and Vanish On You
One of the features I’ve yet to explore in Pokemon: Let’s Go! is the ability to move Pokemon from Go to Let’s Go! Although apparently there’s some danger in doing so, if the process is interrupted in any way. According to Nintendo, moving Pokemon from one game to the other can be risky stuff.
The same support page highlights some proper practices when performing this move, in order to prevent problems like vanishing Pokemon from occurring at all. Make sure you avoid pressing the home button or the power button on the Switch, pausing the app, pressing your phone’s home button, or turning off Bluetooth on your phone. You also want to avoid having too much distance between the Switch and your phone.
I don’t know first hand what the benefits of sending over Pokemon are, although there’s XP and candies involved if you perform the process. There’s a park complex they’re all sent to in Let’s Go! that I’ve never been to. Perhaps it’s quite lovely there. Maybe you run around, greeting pocket monsters and soaking up secondary benefits. Maybe they’re in cages and being fed by a series of tubes. It’s to you to find out for certain, as I am 100% never installing Pokemon Go on my phone.