When I export font projects to either TrueType or OpenType formats, the filename gets the OTF extension regardless. I’m not sure if this rises to the level of a bug (the font seems to work OK regardless), but it’s an unexpected behavior.
The filename is set in the Export Settings dialogue, and can be other *.ttf or *.otf.
The filename extension is inconsequential, as you discovered, since either will work just as well.
I prefer to use *.otf for my fonts since they contain OpenType features, but OpenType fonts do not have to contain OpenType features.