@Paul S.
I've figured out a work around, if you change your 'Settings' > 'Display Settings' > 'Temperature Unit' from F to C, you can then change the disk settings.
I am also having this problem. I have a couple of NVME SSD's in a cache pool and cannot change temp thresholds. Have tested with Firefox, Chrome, and Safari. Delete Pool also doesn't work.
Running 6.9.0 Unraid OS Pro