Game version: 9002
Operating system: Win 10 64
Tiles or curses: Tiles
Mods active: None (DDA Only)
Activating RM13 Combat Armor does not change its weight.
Activating RM13 Combat Armor increases is weight to over 400 kg. The weight increase persists even after deactivating it.
Start new game. Spawn RM13 Combat Armor. Spawn plutonium cells. Reload suit, wear suit, activate suit.
I can confirm this behavior.
Can this be a mod issue? I can't replicate it myself.

... ah, I see, it happens a turn later.

That is really bizarre.
Ah! This bug might also be affecting Advanced UPS. Does the RM13 take plutonium? [edit] Ah, I see it does from the screenshot[/] Or just have charges?
My advanced UPS jumps to 120kg. :P
Seems the recent linking of charges somehow is linking Plutoniums weight to the number of electric charges, about 80 grams each? (Yep, Advanced UPS holds 1500 charges and is around 120kg and some change)
Seems Plutonium cells are 40 grams each. Confusing.
The shift in weight is probably caused by this

Which means it comes from #30255
Yeah, checking files instead of wiki shows 0.08 wieght of the Plut cell. So I've quick fixed in my own game by just making the cell weigh 0.00 for now. :P
Ah... game don't allow that. :(
0.1 seems to be the lowest I can set without an error.
Most helpful comment
The shift in weight is probably caused by this

Which means it comes from #30255