bitcoin-qt ignored user input for fee calculation My blockchain isn't synced yet. Thus, fee calculation didn't work. Therefore, I chose user defined fee and entered what btc.com suggested.
But my bitcoin-qt (v0.20.1) went for 255 satoshis instead of some 10 thousands or so. (A rough guess, since I can't recall the actual amount any more.)
The question is: Did I encounter a bug or is there some hidden magic involved?
I believe you're confusing units here, and entered a value intended as sat/vB in a place that expects sat/kvB.
Perhaps you intended to use 40 sat/vB (=40000 sat/kvB, or 0.0004 BTC/kvB), which would translate to around 10000 sat (or 0.0001 BTC) for a typical transactions.
When interpreted as 40 sat/kvB, it would be far below the minimal relay fee on the network (1 sat/vB, 1000 sat/kvB, 0.00001 BTC/kvB). The sender software you used may have rounded it up to 1000 sat/kvB instead, and used that - which may translate to 255 sat for a typical transaction. That is far below the current blockspace market rate, and may explain why your transaction hasn't even propagated (blockchain explorer sites don't even show it as unconfirmed).