zstash is always non-blocking. Reported by @golaz:

Running zstash create -v --hpss=globus://nersc/home/g/golaz/2023/E3SM/fme/${EXP} --maxsize 128 . results in non-blocking archiving. This is unexpected since the --non-blocking option isn't included. That option was introduced in #214, which was before the last release (zstash v1.3.0).