It is expected to have syncs during backups if they are set on local drives under VM - as this process generating a lot of IOPS for single time on day, and Zerto handling this by Bitmap syncs after buffer fills.
It is always recommended to perform backups to a dedicated NAS or storage subsystem in different network, so if something happens with VM - backups will be not affected. But if it's not possible to create architecture like that, and you wanted to have low RPO values at any time.
However you can apply Temp data storage, on which all I/Os during your scheduled backup will be ignored and this volume will be not replicating to other side - this can be useful to avoid unnecessary syncs and alerts.
Using this setting will ignore all I/O on marked disk, and all changes to it, as Temp data disk transferred only once to DR site, and all further changes on this disk will be ignored.