Hi,
I've upgraded Penumbra to version 0.4.4.5 after Dalamud's major update, and I've been having rather frequent issues of "modded models not appearing" since then.
I think the affected mod is "The Unofficial HrBody 2.5" for hrothgars (with 4K textures if that makes a difference), you can find this mod here.
The issue happens most often in the pop-up when I try some gear on. It also sometimes happens in-game with other players and myself.

The mod structure I have is :
- The mod is loaded on my character collection with all the bells and whistles (HD models)
- The mod is loaded on the default collection with only the textures enabled so they are the same between me and other players/NPCs but keeps the performance hit to a minimum
Even if I try to close and re-open the try-on window, the problem re-occurs. When I saw the problem on another player, I tried redrawing the models manually with /penumbra redraw, but the issue persisted when the character reloaded. If I play around with the mod settings (such as disabling mods/collections and whatnot), it tends to fix the problem until the next loading screen.
I don't remember seeing this issue on Penumbra 0.4.2.7, so there must've been some kind of regression somewhere.