If you temporarily disconnect in Add-ins PDM and this problem will not be, it will mean that Property Manager and PDM use differents methods of working with properties.
In PDM it is impossible to open the card on several files - probably a problem in it, and it will have to consider.
Moreover, it is probably the smallest problem and if you want to use Property Manager and PDM - from my experience - is better to select only one of them.
Also files can be in different statuses. There can be a reason in it?
Disconnecting the PDM add-on is not an option and selecting multiple parts at a time is a huge time saver for us [we use Solidworks and PDM differently than most companies]. I submitted the issue to my support company and they in turn created an Rx and submitted it to Solidworks R&D team for further review. Thank you for your input though.