Same problem for me that LUT files are grayed out and cannot load them in Photo for iPad v.184.108.40.206. running on an older iPad Pro (12.9") with iOS 12.3.1. Tried files on Dropbox and local on the iPad. Same problem either way.
I am experiencing this issue also. Twice today alone the app stopped responding for very long time when I tried doing a "Union" or "Add". The first time, I waited for quite a while and then the system said my memory was all used and that I had no disk space left! I ended having to shut the computer down with the power button. When I restarted, I looked and I still have 50GB free. Hope this gets fixed also.