I'm still going with bacterial for now. It's not uncommon for this group of rainbows to not respond to furan. That's happened before.
Another fish had suspected columnaris and it took two rounds of kana and furan to even get the cotton to just go away, no less actually having the wound heal.
They have a tendency to get some very nasty and some extremely difficult infections to clear up.
Needless to say I'm not getting anymore rainbow fish ever. They're just too much hassle and harbor way too much bacteria, some of which is hard to even diagnose. There's been days I gave up sleep to do a water change, so there's no way it's lack of pwcs. You can't tell me it's my water when I have three filters running and three bottom feeders who literally live amongst the poop if the tank is dirty. The pictus have never been sick, not even once. The synodontis has gotten fungus in the past but nothing awful recently. I've had tetras live PAST their expected lifespans aka longer than five years. In addition to that, I've reversed hole in the head disease.
The tetras only began getting sick once the rainbows came. In fact, this is so annoying, I'm not adding new fish until all of these dither fish die. Rainbows are hardy, yes, but for anyone who considers them, I am of the opinion they need a species only tank after having experienced this ridiculous saga and sleepless nights of worry.
Secondly if they're this difficult to cure that means they had to be heavily treated when they were for sale.
I don't have the heart to bring them to the pet store which accepts fish because they have dead fish constantly.
For the record though they're the only fish I've ever had who seem like they "always have something".