This Article explains the behaviour when junos only upgrade is performed on a VM host RE on PTX.

When trying to perform a Junos-only image upgrade on an incompatible VMHost OS, e.g., perform an old Junos-only image upgrade on a new VMHost OS or perform a new Junos-only image upgrade on an old VMHost OS,
there is no compatibility check, and the Junos-only image upgrade will still go through. 

The system might go into a bad state if the Junos-only image is not compatible with the existing VMHost OS.

There is no compatibility check when performing a Junos-only image upgrade on an incompatible VMHost OS.

Please do not perform a Junos-only image upgrade on an incompatible VMHost OS. It's better to use a VMHost image for the upgrade.

For More information refer to the below link.