If an item frame is on a block, where a comparator would normally be able to read the item frame from the other side, when the frame is in certain blocks, such as a repeater, that doesn't break off the item frame, then the comparator will no longer read the item frame and when updated, will ignore it. 

Note: In the attached video, updating the comparator with a repeater gave the same results as with a block, multiple times. 

Has not been tested in any version other than 1.14.4, but I wouldn't be surprised if it was present in other versions.