Hello,

I've had this Thai constellation and these brown spots under the leaves keep spreading.
The plant is still putting out new leaves and I have replanted and there was no root rot.
I have been checking for pests and haven't seen any, not to say I couldn't have missed them

That really does look like Thrips to me. The brown residue is their poop. Can you wash it off. Or is it adherent and hard to remove? Thrips are hard to see, you might need a magnifying glass, and with a pattern like the variegation on your juvenile TC, the damage might tend to blend in.
If the brownish stuff is adherent and won't come off, it could be a mold/fungus.But it really looks like thrips to me