Action Bars 7-10 are duplicates of the 'Action Bar 1' of different forms. Dragging an ability off of Action Bar 7 will result in the exact same change in the 'Action Bar 1' of cat form. Removing or adding an ability from Action Bar 1 of cat form has the same effect on Action Bar 7, etc.

Each Action Bar from 7-10 is a duplicate corresponding to the Action Bar 1 of a different form:
Cat - Action Bar 7
Tree - Action Bar 8
Bear - Action Bar 9
Moonkin - Action Bar 10

Most testing was done as a level 110 resto druid acquiring Moonkin form from the level 45 talent Balance Affinity, and acquiring Tree form from the level 75 talent. Balance spec also shares a duplicate of Action Bar 1 and Action Bar 10, so I suspect that this problem presents on the same bars regardless of specialization.

Edit: While Restoration spec and selecting Balance affinity, the Tree of Life form uses the stanceless (humanoid/caster form) Action Bar 1 instead of a duplicate of Action Bar 8. A duplicate of Action bar 8 is used for Tree form if Feral Affinity or Guardian Affinity is being used.

So, this isn't actually a bug. What you're experiencing is bar mirroring, and you can mess with the settings for it via the action bar's right click menu under paging settings.

Thanks for taking the time to reply! I played around with the paging section of the action bars and was not able to find a way to free up any of the bars from mirroring. My default settings for the druid stances were all 'Disable' to begin with. I did not see anything regarding 'mirroring' specifically. Altering the paging settings seemed to only change which bars are mirrors for each other. Is there a single option to disable bar mirroring altogether?

Are you looking at action bar 1?

Some, or perhaps most of the time, the mirrored substitutions were made on action bar one. But, in a few cases, when I messed with the paging settings, the action bars 7-10? mirrored on action bars other than number 1. My ultimate goal is to have 6 or more action bars that are truly unique, but I haven't been able to find a a way to achieve that.