Drones can lose the ore they’ve mined in some situations, from what I read it happens in situations like if you give new commands, if the drones attempt to deliver back the ore and your cargo is full or if you get attacked and pull back the drones.
This has been like this for years and as someone who is just a player I cannot easily think of a fix that could stop these situations from happening besides something like ‘send ore back to the asteroid whenever one of all these situations happen’.
Anyway, the easy fix is to not use mining drones on mining missions. They’re only a fraction of your yield anyway.
I don’t think this is about residue, as far as I know residue doesn’t happen on mission ore?
What’s happening is a different bug that has been around for a while now, like this one: