Has the default minrelayfee in Bitcoin Core config been updated to 100 sats/KB or something?
To make it that value or lower for a newer version of Bitcoin Core is the only way that these sub-1sat transactions would be allowed to propagate, or else they would have been discarded by the nodes.
That, or Bitcoin Knots made that adjustment.
I believe miners are free to set the minrelayfee themselves, and their reasoning for choosing the particular low value is based on the assumption that it's better to mine the chock-full blocks rather than half-empty ones. That's why we're currently seeing blocks filled with those sub-1 sat/vB transactions. One of such block is
906805 which is filled with the sub-1 sat/vB transactions containing inscriptions.