VSS shadowstorage resize failure

To free disk space, attempted to resize shadow copy max. storage space using:

Command Prompt (admin) $vssadmin resize shadowstorage /on=C: /for=C: /maxsize=15GB

The command appears - at first - to pass successfully, i.e., allocates max. of 15 GB for said purpose.
However, few days later, or possibly after system reboots, command gets overridden and re-allocates
50GB instead of specified 15GB.

This is the 3rd attempt around.

Any thoughts what could be the cause(s)?

Yes, Avast (spooked )

$vssadmin list providers

Provider name: 'Microsoft Software Shadow Copy provider 1.0'
Provider type: System
Provider Id: {b5946137-7b9f-4925-af80-51abd60b20d5}
Version: 1.0.0.7

Much appreciated. Thanks for pointing to the Avast link.

I erased all previous stored shadows via $vssadmin delete shadows /all
to circumvent any potential corrupted copies.
Then set a max. allowed storage of 10 GB.

System has undergone a reboot since, without affecting the setting.

Used Shadow Copy Storage space: 440.563 MB (0%)
Allocated Shadow Copy Storage space: 2.188 GB (2%)
Maximum Shadow Copy Storage space: 10 GB (10%)

My observation is that as soon as stored copies approach max. allowed volume the command will
get overridden.