AFAIK the balloon driver will not actively release ballooned memory. It will deflate on request, i.e. when the guest OS requires more memory, and there's sufficient free memory on the host. Migrating (vMotion/DRS) will not help. The easiest way to release the ballooned memory is to reboot the VM (if this is possible in production).
Anyway, since the guest doesn't seem to need the ballooned memory for other processes, you shoulbn't see any issues, and may delay a reboot until the next patch day.
André