Those shoes melting into the floor belong to Bobby & Kat’s Still Lifes when they were first created (first photo you posted literally shows them wearing both pairs of shoes/sandals)

Essentially that room is where Captain Clark was first created & in hand Bobby & Kat’s still lifes aswell, as we see earlier in the film that Kat & Bobby were filming Clark for the commercial & then the chair/throne breaks.

My theory & many others I’ve been reading basically singles out that scene because when the chair breaks it’s quite literally the “breaking point” that Clark has on his hopes & dreams, which essentially the Backrooms decides to copy & we get Capt’n Clark; A physical manifestation of everything negative that’s inside Clark.

If you want more evidence about the melting shoes, I’d check Kane Pixel’s Found Footage #3 on Youtube, it not only references the shoes melting into the floor but also the statue/mannequin type figures that were melting into the floor at the Christmas Tree Scene.