You're on to something because this issue does not occur if I export as v4.0. I added extra keys as you suggested, but unfortunately it's not solving the issue for v3.8.

I'm also baffled because the rotation is less than 360 degrees. I'm going to keep working on this, so let me know if you have anything else I should try.

When I export an animation as a Json file version 3.8 the resulting file loses any rotation information. I am a complete newbie to animating, so am I missing something in the file setup?