I'm working with a PC (LR Classic 10.3, LR 4.3) and an iPad Pro 2021 M1 and latest LR mobile. I had the problem, that collections that were edited on iPad have been synched with LR, but not with LR CC. So I tried your troubleshooting with deleting the whole cloud library and synchronising it new. Although I lost the on iPad done editing work of two shootings. But the problem were solved. Now, about 2 or 3 weeks later, the problem is back. Half of the collection I edited today on iPad did sync, half did not.
Really: This is no fun. Yeah.. LR mobile on new iPad Pro is awesome. But when my work on iPad is a waste of time once a month, that's it. I need a software I can REALLY rely on. Especially when I pay a monthly fee for it.