Update and Replace Cache not working. Preventing me from merging two schematics. (17.2)

I have a design where several subsystems are coming together into a single design, and each subsystem was created at various times in the past few years. As our library has evolved over that time, the subsystem project caches have become out of date with respect to the library. 

So, in order to merge all of these subsystem projects into a single, new schematic, it's my understanding that I'll need to perform an Update Cache in each subsystem project to bring them all up-to-date. 

This doesn't seem to be working, however. I've also tried using the Replace Cache (Preserve schematic part properties), but there doesn't seem to be an effect. Note that I've tried using these two functions on both sides of the merge, meaning I'm trying to update both the to-be-merged project and the to-be-merged-into project so that they're both updated with the latest library info. Note that I don't want to use the Replace Cache - Replace part properties option because that blows out all our PN information, which I'd have to go back and painstakingly add. 

I've tried all kinds of combinations of Update, Replace, copying into a new schematic, and Updating All Part Status in the Part Manager. Nothing works. 

Most strangely, there was one time where it DID work. I just had to Replace Cache (with identical path) for a few parts and everything copied over! Exactly how I'd expect this should work. 

I need help here. This has been a thorn in our side for a long time, and we haven't been able to figure out what's going on. 

EDIT - 

I just found that if I Replace Cache with a temporary part, and then Replace Cache back to the original part, it all works. For example, I can Replace Part "CAP_1 : \\PATH\\" with "CAP_2 : \\PATH\\", and then immediately use Replace Cache to revert back to "CAP_1 : \\PATH\\" and then the copy of the schematic page into the new schematic works. Something wrong with the Update Cache / Replace Cache (with identical part). There're no error messages or other indicators of anything going wrong during the Update Cache function.