You're absolutely right. It's repeatable. And, if you take the next set and try to set it to 78 pixels again, it'll go up to 632 pixels!
BUT, I found that ONLY happens when using an Artboard - it works like normal, if you create a document and don't choose "Artboards" in the New Document panel.
I will be entering a bug report here (Feature Request/Bug Report Form) today. May I suggest you do so as well, so that they can see it's repeatable?
The post I created is at: Problem Resizing Canvas when Artboards are Used | Photoshop Family Customer Community on that post, you can add a Me Too at the top of the page...
Just an aside, by the way - someone on that other community suggested using the Artboard tool, instead. I'd failed to ask you, @stevecrown, had you tried that?
I still say though - if we can make documents that are artboard based, those *should* work like canvases... we should be able to resize like normal. I'm going to leave my post there, just to see if anyone else has an idea, or if Adobe checks it out as a feature request, if that's not currently the case.
Didn't try that. Makes sense that the artboards aren't necessarily to be used as canvas's, but still an odd issue.
Adjusting them using the artboard tool seems to work though.