First, check the Save command in the File menu and make sure CTL-S is still showing up as the shortcut. Can’t imagine it wouldn’t be, but you never know.
Second, if you do anything that causes the image to be anything but a flattened document (create a layer, make some text, add an adjustment layer) Photoshop will default to trying to save as a layered PSD to preserve the layers. Even if you only have one layer, if it’s not a flattened background layer, it’ll try to save to PSD.
Since you already have a save-to-JPEG Action started, you could add a “flatten” command to the start of it and that should clear up your issue.
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It is easier to destroy than to create.
More fun, too.