You have a tricky one here. Try the following. In the Data > Connections, try to reset your connection. I know this looks like the "reboot computer strategy," but sometimes the connections take time to update, and things like this happen. I can't tell you how many times I fixes similar strange issues like this by working on the connections.
Another trick is to export and import the Flow again. This will force you to define the connections and reset them, and this may also solve the issue.
Let's try this and see if it works. If now, let me know, and I'm sure we can figure this one out.
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