What the fuck is a 'Pod Archive' and why does it replace my textures instead of converting them to flats?

This probably doesn't help much but I'm pretty sure it's only a SLADE bug displaying the wrong lump type and icon. Try saving it as a new file then re open the saved file, does it change to Doom flat?

Paging @Kappes Buur he would know, I hope.

I had the same thing as I tried to recolour some light flats after new slade update. 

It's a bug in the entry type detection code.

Pod archives are IIRC the archive type for Terminal Reality, Fury3, and some other games (here's a list if you're curious). Obviously your Doom flat is not actually a valid pod archive, so SLADE is bugging here.

Try changing your markers to F_START/F_END then reload the wad.