roots look eaten, can’t find the culprit

i got this guy root bound, growing out of the holes on a 4” pot, and struggling. i didn’t realize i’d run out of 6” pots, so i put it straight into a cover pot (it has a hold). today i was going to move it into a nursery pot and found the roots are just… GONE! there’s no trace of them in the soil. i was expecting some root loss since the ones that’d poked out were kinda dry, but there’s nothing at all! the ends look like something has eaten them and there’s a lot of fine powder that looks like “sawdust” around what’s left. i can’t find any kind of bug tho! i spread the soil out and haven’t seen anything move, not even tiny things. i rinsed the plant in a water:alcohol mix in hopes that if its hiding up in the rootball, it dies. and to remove any remaining soil. i’m planting it in a smaller pot again with new soil.

anyone seen this before? i’ve had it for a couple months maybe and recently noted the lack of growth. ideas? things i should check or do to help it recover?

more details:
– i found a centipede in my fern while repotting it last night. it was on a different shelf, but would those do something like this? i’ve never found one before. coulda been from the store.
– in my moss and perlite rooting tray i’ve also found pinprick size, round, black or translucent bugs that haven’t ever been identified. they eat holes into leaves, but the spot usually turn black and they seem to prefer thicker leaves. they’re the size of the sawdust, and it doesn’t move…i checked 😔
– most of my plants are in covered/enclosed shelves in front of my south facing patio doors. on top of the sun, each shelf has a tube light or 2. my humidifier keeps it ~55%. – spider mites are the most common pest i see, but i’ve seen an uptick of thrips lately. those are both leaf pests tho.
– i definitely get a lot of outside bugs inside cause county housing contractors suck. it’s usually just spiders and flies tho, and i keep the spiders for pest control.