The way that the transaction is represented on the graph can be quite confusing. Johoe's mempool likely didn't drop the transactions and if it did, you'll see the proportion of 1sat/byte transactions decreasing faster than the rest.
It actually dropped, they've dumped quite a ton of them, and the 1sat/b was the only one affected.
That said, yeah, Johoe's layout is not really my cup of tea.
As for OP, I can tell you what happened, a ton of ordinal inscriptions that while small in number of tx large in size pushed a lot of older transactions out of the mempool that was just clearing a bit up, so suddenly every single estimator went nuts, people stopped sending 4-5 sat/b tx and this discouraged rebroadcasting of older ones so that's why the slow build-up back for 1/sat b in the mempool.
To be honest, is understandable, you waited 3 months rebroadcasting your old tx over and over, and just when you see the light you see another wave that pushes you again out of all 300MB mempools of course they would give up just CPFP so highly probably a few of the 1sat/b have just been upgraded o 4-5 or even 10sat/b as some lost patience. I would have probably done the same, just pay 5-10$ and get it off my mind.