I work with 16-bit Images too, and Scale to match my Project (am always doing this in PrPro, but it's the same). I always include a line in my Action to change the Bit-Depth to 8-bit, before doing the Save_As.
It would be the same, if I was working with CMYK Images (only do those for delivery to a printer), in that I would have a line in the Action to change Color Mode to RGB.
Glad that you found the issue and good luck,
Hunt