When wearing RM13 Combat Armor, player encumbrance doesn't update a turn after activating, only updating after the player wears/takes-off something else. This goes both ways, allowing the player to have low encumbrance when off, or high encumbrance when on.
Probably caused by the performance update that reduced updates to player/npc encumbrance. This may extend to other activated clothing such as the riot helmet with facemask, or containers that change encumbrance when holding things like waterskins or magazine straps.
When the armor is activated, encumbrance updates immediately or one turn later.
In previous versions, it would update one turn later either because encumbrance was updated at the end/beginning of every turn, or to stop the player abusing the armor by activating it for less than one turn, gaining its benefits without using fuel cells.
This also happens with normal power armours, by the way.
I can confirm that this is occurring with RM13 combat armor on my game, build 9346. I found this issue report because I independently encountered this issue.
I would also note that it appears to update the encumbrance when z-levels are traversed.
I would also note that it appears to update the encumbrance when z-levels are traversed.
It also appears to update encumbrance when you toggle a bionic on or off.
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This also happens with normal power armours, by the way.