Since initially creating this post I created a case with Adobe support as this may be a bug. Let me know which is the appropriate channel and I will close the redundant one.
In 6.2 we were able to select multiple pages and then properties to allow bulk editing. The path browser option to select tags was active regardless of the values of the currently selected tags for the pages. The common tags were displayed and any new tag selected was added to the tag set for each of the pages.
In 6.5 the behavior has changed as the path browser option is inactive unless the tag sets are equal across the selected pages.
There does seem to be a work around - uncheck one of the pages and then recheck. This results in the path browser button being active.
How can we allow the behavior of 6.2 in 6.5 without the work around?
I tried reproducing this in my local AEM 6.5.0.
Though the tag field is editable upon removing one page from selection and including it back, associating new tag will not be persisted upon "Save & Close" (No input on cq:tags field in POST request)
It should have been an intentional behavior(Not allowing to edit/disabled tagfield/not persisting the data even if it editable in the UI) when the tags are not exactly/completely equal in the selected pages.
Given that you already raised a Support ticket, you can get this affirmed from Engineering team.