Well I found it with a support call and a second set of eyes. His first thought was security as well but that wasn't the issue. It was actually two unrelated things that made it appear to be the same issue.

In the first scenario, in my original post, we were searching a date range in the Post Date field of the Journal Entry template. The issue was that when we copied the Process from our Test Forms environment to the Production Forms environment, the "Save To Repository" task stripped the Template value out of the Parent Folder fields, so the metadata fields were being populated, but without a template selected. Since the search syntax is specifically looking for the Post Date field in items with Journal Entry selected as the metadata template, and no template was assigned, the results were empty. The fix was in the Process in Forms; nothing wrong with the search syntax.

The second scenario was a workflow that adds metadata (template and fields) to new employee records after they are hired. A new employee has a number of documents to upload via forms, and when reviewed and approved by HR, they are placed into a folder named after their name and employee number. It would then trigger a workflow that would search for that folder and add the "Employee Records" template to it and apply the correct metadata. The Search Syntax in that workflow also contained "{LF:Name="*", Type="F"}" to limit the results to folders, which is what made me think that was the broken part. Turns out the problem was that the employee had an apostrophe (or single quote ' ) in their name, and the workflow wasn't designed to escape single quotes when parsing tokens. So the %(LastName) token would go into the search as "O'Malley" (for example) which borked the search. Needed to be "O''Malley" (double the single quote to escape it and use it as a literal single quote instead of a string encapsulator). Just a huge coincidence that we hired someone with an apostrophe in their name at the same time as the new Journal Entry process was being used for the first time since we deployed it. I fixed the workflow by replacing the "Search Repository" activity with the more efficient "Find Entry" activity, since we know the exact path to the folder we want to operate on anyway. (I built the workflow a few years ago when I wasn't as familiar with all the activities available to workflow)

Thank you to Dustin and Miruna for taking the time to read and respond to my posts. It's great to be part of such an active community!