You can find out what has changed in Acrobat (at least the things that were deliberately changed) in the release notes. They also list a date of when the update was released: https://helpx.adobe.com/acrobat/release-note/release-notes-acrobat-reader.html
You very likely wont find somethink like "broke behavior when saving to a shared folder" 🙂 But at least this gives you some dates to work with.
When you say "shared folder", is that something like DropBox, OwnCloud, OneDrive or similar service? If so, you have another player between the document in the folder and Acrobat. I've seen odd behavior in similar situations. The problem here is that the application (regardless of whether it is Acrobat or any other application) does not know that the file is not stored locally (or not only locally), and that the DropBox/OwnCloud/OneDrive... application is making a remote file look like a local file. Sometimes that does not work 100%.
Tested different settings and found a solution, just in case someone else has this issue in the future. Unchecking the box/setting "Enable Protected Mode at Startup" under Security (Enhanced) preferences seemed to resolve the issue. We no longer receive the error message and the file does not get turned into a tmp file when saving in the same shared folder under the same name. I know this is the same setting that gives another issue with CommonLook plug-in.
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