Sounds like that's worth checking but I don't think that's related to Alexanders change: Even before Adwaita applications were supposed RRECOMMEND gnome-icon-theme if they needed it (not saying it's necessarily a good idea but that's how it used to work and still works). So if pcmanfm doesn't do that, it may be missing icons already and did so before adwaita as well (assuming nothing else in the image pulled in the icon theme).

I don't think it's likely but there is a chance that gnome-icon-theme contained icons that Adwaita does not provide (comparing them all turned out to be tricky). So double-checking how pcmanfm looks with Adwaita might make sense in any case.

Jussi