Slade - Converting to Flats Problem

This is an odd one i haven't wrapped my head around.

Using Slade, when i try to convert a 64x64 PNG file into a Doom Flat, sometimes it will convert successfully, however sometimes it converts to an 'unknown' file type and i have to delete it and try again since there's no other way to rectify the now essentially corrupt file.

The images are between F_START and F_END. The base resource and palette are both Doom, so why does this happen? It's a bit odd that it seems to be a dice roll to whether or not it converts properly.

Edit: Figured it out almost immediately. It seems the flats only convert correctly when they're placed above an already converted flat and not under it. Why is this a thing? Despite solving the issue it still raises some rather odd questions.

When you say the file is essentially corrupt -- are you sure? If its byte dimension is 4096 it should be a flat.

I say that because it's important to figure out if the issue is in the conversion routine as you imply, or merely in the identification routine...

Well, it's merely an assumption that it corrupts just because Slade can't identify it.

When it converts to 'unknown' the entry size is 4096 bytes. The entry type is unknown. As i say though, this only seems to happen when the image is converted under an already converted flat.

I've had this issue before too. The file is not corrupt since even if the type shows up as "unknown" on SLADE it works in Chocolate Doom.
The only way I've found to fix this is the following: Once you've converted the flat and you have this problem, just close the WAD and open it again. If that still doesn't fix it, close SLADE and start it again. That usually does the trick.
If I was less lazy I would look at the source code to SLADE and send a pull request (SLADE uses Git, right?) to fix it.

Yeah it's definitely a glitch in the identification code.

Flats are only identified by their size and their presence in the flat namespace -- there's no simple and fast check one can make to verify that a data blob of 4096 bytes is truly a Doom flat. When a lump is modified, its previous type is removed and it goes through identification again, and so it looks like it fails to see it's in the flat namespace.

Ah, i see.

So it does convert correctly, it's just not identified properly. I did try closing and re-opening applications before but it didn't always work unfortunately. At least now the issue is apparent.