Hello Advanced Member 😉
The usual advice is to set the Minimum for each user share to larger than the largest file you expect to write to the share.
Unraid has no way to know in advance how large a file will become when it chooses a disk for the file. If a disk has less than Minimum, it will choose another disk.
Note that doesn't mean the space will not be used. Here are some examples:
A share has 10G Minimum, a disk has 11G free. The disk can be chosen. You write a 9G file to the disk. It now has 2G free, which is less than Minimum, so the disk won't be chosen again.
A share has 10G Minimum, a disk has 11G free. The disk can be chosen. You write a 12G file. The write fails when the disk runs out of space.