Edit TLDR: hot mud to fill void behind vanity didn’t completely stick. I noticed a crack and found that there’s a gap between the newly applied mud and the wall. The big hole is what I cut out to investigate the crack. You can now see that gap between the mud and the wall, which is my underlying issue.
Painted this bathroom with SW Duration, then installed the vanity a few days later, which brought to my attention a 3/8” gap between the vanity and wall. Wall was cupped.
I sanded the wall and wiped it clean, then used Easysand 45 to build up the wall to fill the gap and give a solid backing for my backsplash. I did a pretty thick layer at just under 3/8” thickness near the bottom and feathered out at the top. Then some sanding and additional beautification layers spread out over a few days. Today, on my final sanding pass before primer, I noticed a crack and started cutting into it. As you can see, the hot mud didn’t adhere where it’s the thickest.
What did I do wrong? Was the paint not cured enough after ~4 days? Did I lay down the hot mud too thick? Any suggestions appreciated.