A fix for this issue has been published with vCenter Server 7.0 Update 3o.
Should you be still experiencing this issue with this or a newer vCenter version, please contact the VMware support by opening a support request.
Workaround:
As a workaround you can reduce the agressiveness for Storage DRS by setting the "minimum space utilization difference" value to a higher percentage, as shown in the image below.
This will result in SDRS applying a more conservative strategy when balancing datastore usage, leading to a lower number of Storage vMotion tasks and allowing vCenter to execute these tasks with a far lower risk in them queueing up.