Item Frames Do Not Send Comparator Signals When Submerged

Comparators normally work just fine with item frames, outputting a signal strength based on the rotation of the item. When you submerge the frame in liquid, it doesn't send any updates until you rotate it once (or if you block-update the comparator) and it just shuts it off.

You can not draw a signal from a submerged item frame, and I haven't seen anywhere that states whether or not this is WAI or a bug.