Thanks for taking a look. I did note (see my 2nd sentence of my original post) that I, also, cannot reproduce the problem. It's only occurring in situ. So, it's more a question of whether or not anyone has run into the same or similar issue, and what they discovered about it.

I'm also happy to provide the original program where the problem is occurring, but at 20k it seemed a bit long to post unsolicited; also, there are external conditions for running it, such as interacting with a active AWS/EC2 account. But anyone as curious about this as I am can just ask, and I'll share the full program where the problem manifests.