UK eSIM Spusu - "Invalid Sim Card"

Spusu is a great deal, but unfortunately about once a day my phone says "Invalid Sim Card" and the mobile network stops working. Airoplane mode doesn't fix it, but a reboot does. Then it comes back again a few hours later or the next day.

Using an esim on a Pixel 5. the internet suggests the issue is because the phone is stolen or has a fake IMEI, but I've never had issues with any other mobile network, esim or otherwise, and besides why would they let you use a stolen phone for a whole day before blocking it?