This is indeed a bit puzzling. My theory is the following:
Your brother's computer running a mining node was somehow only connected to some other node, but both weren't properly connected to the rest of the bitcoin nodes out there. Your brother's node started to mine on a fork block which was confirmed by the connected node but not by the rest of the bitcoin network.

All further mined blocks of your brother's node were chained to this fork block which the other connected node happily confirmed. Maybe that other node also contributed some blocks when it mined blocks itself.

Because those two isolated nodes didn't talk to the rest of the network their false diverted leaf chain never got "corrected". This only happens now when you rescan and reindex the blockchain.

I can't come up with another explanation why your example in the screenshots show 437 confirmations which then disappear.


Regarding the debate which lock scripts control the potential UTXOs. I wouldn't care too much on this. I'd be more interested in the transaction IDs, which must be known and existent in the current true blockchain, if the mined UTXOs were real.

When I look at the speed of generated coinbases in your screenshot, it looks surprisingly fast to me, if the generating wallet were connected to the real bitcoin network.

Does anybody know if the timestamp display of transactions in Bitcoin Core is affected by your current timezone?
@gigi0ne, in which timezone are you?


Dave was faster than me to respond...