SharePoint Doc Library Folder - ValidateUpdateListItem not working anymore

[SSA4]

I have used this wonderful trick on many sites over the past 2 years, but for some reason – on a site that I swore it used to work on, I can no longer update the “Author” field this way.
– It’s a one-step Flow. “For the selected file” > “Send HTTP Request to SP”
– It’s the same code I use successfully *still* on other sites in our cloud environment.
– I’m only editing the Author field.

– I have “bNewDocumentUpdate”:true.
– All my quote marks are double/single appropriately.
– I get no error message. It says the flow ran successfully.
– It’s for a Document Library. And it doesn’t work on Files or Folders. (I want it to work for Folders, but tested on Files too.)
– If I change to “Editor” field – it works. It just won’t work on “Author” field.
– I’m a site collection admin and it’s running on my account.

Anyone have any other ideas of what the glitch might be? Thanks!

[Spinner]

I have the opposite problem: can no longer update Editor, but can update Author without issue. (Also can't update Modified date.)

[SSA4]

I'm still having this problem. It's sort of all over the place. Works on some site collections, not on others. In some Flows, but not others. Sometimes it's the Editor. Sometimes the Author. Sometimes the Modified Date. Honestly, I forgot I had posted this - I had just given up on it back in January and have been living without.

[Spinner]

Bizarre. At least that suggests it's not the code. What about connection references? Could it be some weird SharePoint permissions issue for the reference used by the flow? 

[SSA4]

It should be using my permissions, and I'm a Site Collection Admin - so it shouldn't be a permissions issue.

[Spinner]

Same here, but I'm struggling to think what outside the flow (which seems fine) could be interfering with it.

[SSA4]

I only thing I don't have access to that I've wondered could be the source of the issue would be a setting at the Environment or Admin Center level. If it's a setting across our entire Cloud. Perhaps a File Retention setting that conflicts with it. Although why a global setting would affect different flows differently, I don't know.

I have to confess that I haven't looked at my Flows that use this in months. For all I know, none of them are editing the Author, Editor or Dates at all at this point.

I also wondered if it was older Flows that were written before some Microsoft-level update, those work, but newer Flows written from scratch don't. [It seemed like this to me, but I can't be certain it was this clear of a divide.] Some of my Flows are 2+ years old and the Flow interface was definitely upgraded by Microsoft at least once after that. Because Flows hide so much of the "real code," it's tough to troubleshoot. Could be a single typo buried in the real code that we can't see in the Flow interface.

Come to think of it, I was hopeful that was what it was and that it would magically "fix" itself someday. I guess that was wishful thinking.