It turns out transferring your beloved critters from Pokemon Go to either of the Let's Go games can be something of a gamble. If the transfer is interrupted, it can result in those critters disappearing for good. This isn't a bug so much as the result of imperfect technology and, based on Nintendo's recommendations, it shouldn't be too hard to avoid.
The folks over at Gamespot picked up on a rather important notice from Nintendo offering tips on how to avoid losing your Pokemon while transferring them between games.
The first Pokemon games for the Switch boast a pretty rad ability to move critters you've captured from the mobile game (Pokemon Go) into your collection for either Let's Go Pikachu or Let's Go Eevee. That should serve as a huge time saver while adventuring out in the wild, and even gives players the ability to take their favorite Pokemon from the mobile game into a fully realized 3D world based on the original Pokemon games for Game Boy.
As Nintendo has noted, though, you want to make sure your conditions are ideal for this transfer, as any interruption will result in the loss of Pokemon that were being transferred. That seems like a pretty terrifying risk considering how seriously some folks take these games but, as the original report points out, there are a few things you can do to make sure nothing goes horribly, horribly wrong. Of course this is modern technology we're talking about, so don't be surprised if we start seeing forum posts popping up lamenting the loss of a favorite pocket-sized monster.
In short, when you're moving critters from Pokemon Go to the Let's Go games, you don't want to do anything that's going to interrupt that transfer. From the Switch side, that means you should just leave the console the heck alone and keep your fingers well away from the Home and Power buttons. On your mobile device, you'll want to avoid the same situations, making sure not to hit the Home button on your phone, suspend the Pokemon Go game, or accidentally turn off the Bluetooth. Just let the transfer happen and then you can get back to checking Twitter or whatever.
Assuming you stop fiddling with buttons for the five seconds it takes for the transfer to complete, everything should go off without a hitch. The important thing to remember is that, if you fail at this simple task, there's a very, very good chance any Pokemon you were moving from one game to the other will be lost forever.
Of course, Pokemon Go is an app, so there's always a chance the whole process will simply crumble on its own. If that happens, Nintendo recommends that you actually go so far as to uninstall and reinstall the app. The Pokemon will almost certainly still be lost, but at least you'll be able to go out and catch more.
Long story short: Transfer at your own risk.