There are a couple of things you might have missed in your setup.
If you are using the default btrfs raid1 for the cache pool, then you only have 128GB total cache space, since it must mirror everything on 2 devices so you only get the capacity of the smallest.
https://lime-technology.com/forums/topic/46802-faq-for-unraid-v6/?tab=comments#comment-480420
Also, if you don't set a Minimum Free space for cache, then it will run out of space. unRAID has no way to know how large a file will become when it begins to write it. If a disk has more than Minimum Free, unRAID can choose it to begin writing the file. Then if the file gets too large you get out of space. If a disk has less than Minimum Free remaining unRAID will choose a different disk (in this case, on the array) to begin the write. So, you should set Minimum Free to larger than the largest file you expect to write. Minimum Free for cache is in Settings - Global Share Settings.
There is also a Minimum Free setting in each User Share which works similarly. If a disk has less than Minimum Free, unRAID will choose a different disk.