Upon detonating a 20 ton very compact core in a test site, I got 50% research towards compact fission weapons. When I placed a 15kt core and detonated it very very far away, the research did not complete. It says a 15kt core and that's the only option I have in the kilos, so no idea why the research did not complete.
My mods include True Nukes and Warheads, Realistic Fusion Power, and previously I had plutonium power, but I removed it in the middle of the game.

Interesting, I've tried that just now and haven't had any problems getting the unlock - what's your full mod list?
Just to make sure, the core you put in was the standard 15kt (not the compact one or anything)

Yes I have ensured that I detonated the standard 15kt core since I currently do not have the compact and very compact versions unlocked. Full mod list is ~125 mods.
AreaPaste, auto deconstruct tresees on resources, beastfinder, evoGUI, flare stack edited, simhelper, SSP, warheads, YARM, 1168 belt icons, advanced portable reactor, advanced solar, adv heat, atomic robots fix 2, auto deconstruct, automatic logictic chests, blanced waterfill, bigger artillery balanced, RFP, concreep, exoskeleton mk2 and a ton of others.
Save with mod list: https://ufile.io/vuzle22g

When I tried it on that save (I got it to load), the research seemed to complete normally, although from what I can tell you didn't have one of the prerequisites unlocked (although I guess that was an older save)...

Ah yes it was an older save. I checked the latest save. I have all the prerequisites researched and have compact fission weapons at 50% progress.
Link: https://ufile.io/wgmt1csq

I did a 20t very compact detonation, and it unlocked the research for me... (I should really have an indicator for which part of the research still needs doing).

Thought I did that, but perhaps I did the compact instead of very compact. Thanks for the clarification. Yes please add an indicator for which detonation(s) still need to complete.

Yeah, it might be a little tricky as GUI's aren't exactly my thing, but I'll have a shot.
You had a very compact in your inventory at the time.

Lol yes I did. Detonated it and boom, researched. Just as you said it should.
Resolution: not a bug, but a feature request to make nuke progress clearer

No problem, I'll look into it, although it may take some time, as I've spent a long time on TN recently, so want to avoid putting any more big changes into action.
Instead I'll probably build something simple to fill the gap for now.

In the next version it will print out what tests you have left to go whenever you select the technology for research. Thus if you forget, simply reselect the research and you'll get your answer.