Backup Exec 20.6 Hyper-V VM backups jobs are slow and display wrong byte count after VHDX expansion

Hello all

After expanding a VHDX of a Hyper-V 2016 VM, daily differential backups which usually had a size of around 50GB are taking longer and job's byte count is around the same size of VHDX expansion (500GB). I'm using Hyper-V 2016 Resilient Change Tracking for backups.

After the job finishes Backup Sets have regular size (around 50GB)

Other interesting thing is that just after a full backup, differential or incremental backups run with correct byte count. I don't know why, after about 1 week job's byte count get back to around the same 500GB...

Any help is appreciated!