Not sure this is an actual bug, but I noticed that after unlocking the modular armor tech (not even power armor), the Mk3 variant started showing up in my inventory filters. Energy shield Mk3 and Fusion Batteries too. Just those though, that I've noticed. The others appear to still be hidden.

Obviously not a big issue but it stood out to me.

Did you happen to change any of the startup options? Just trying to chase down what I did to make that happen, any other info you think might be relevant will help too. Either way I'm chasing it down now, thanks for the feedback.

Edit: Alright I've just tried to replicate the problem and I'm not seeing it. So just a few additional questions if you don't mind.

Are you playing an old map/save?
Are you using other mods that add/change Power Armor?
Are you using an overhaul mod like Krastorio somehow? (unlikely but possible)
Have you used any commands on the current save?
Did you change any of the startup options, as I asked initially?

If all of the above are false than I'm truly baffled as to what the problem might be, but keep me posted if you do find something.

Thanks for the response. Answers below, unfortunately I'm guessing they won't be super helpful.. I'll try to keep an eye out to see if something else comes to mind.

Are you playing an old map/save?

Fairly new map -- I created it post-1.0.

Are you using other mods that add/change Power Armor?

No, I don't believe so. I'll include my whole mod list on this save below.

Are you using an overhaul mod like Krastorio somehow? (unlikely but possible)

No.

Have you used any commands on the current save?

Not on this save. I just checked my command history and still got the warning when I tried to put in a command.

Did you change any of the startup options, as I asked initially?

I don't believe so -- all of the startup settings for this mod are unchecked (which I believe is the default?).

Here's the mods:
- AutoDeconstruct
- Construction_Drones
- EditorExtensions (haven't activated it though)
- Fill4Me
- Infinizoom
- LogisticTrainNetwork
- Noxys_Waterfill
- Power Armor MK3
- QuickItemSearch
- SPM_Better_Roboports
- Squeak Through
- TimeTools
- VehicleSnap
- even-distribution
- far-reach
- flib
- inbuilt_lighting
- nicefill
- reverse-factory
- robot-recall
- stdlib
- vanilla-loaders-hd

Well that clinches it I guess, the only thing that makes sense is one of the other mods is interfering, somehow.
Probably not worth worrying about, it's just the inventory filters and nothing else?

Yep, just the inventory filters. Here's a screenshot: https://imgur.com/a/cf4xCzO

Definitely not a big deal. If I do experiment more and think of something else, I'll be sure to let you know, if just to solve the mystery. :)

Hi there, just wanted to let you know I had a little free time and so I did some testing and found the culprit: Reverse Factory. It makes a bunch of things show up in the inventory filter right from the beginning of the game, not just the power armor items. I'll ping the mod author to give them a heads up.

Awesome, thanks for the heads up.