When I was playing around with Observer blocks in 16w39a, I discovered an inconsistency between the PC Edition and the Pocket Edition of Minecraft - that Observers don't detect item frame updates. This seems like it's caused by the fact that item frames are considered blocks by PE but entities by PC, and since the Observer block is supposed to only detect block updates, this could technically be considered as intended behavior.

However, this inconsistency between versions still remains, and seeing as item frames already interact with redstone through Comparators, it should be possible for them to interact with Observers too and it makes more sense in the long run.

(This last part may seem a bit suggestion-like, so if deemed more appropriate, I would be glad to post to /r/minecraftsuggestions instead. The main bug here is the feature imparity, and it's really up to the developers on how they should deal with it.)