I made this comment in another thread, but in think i should just create a separate topic

I am using Aseprite 1.2.18 on Windows, using add Inbetween Frames every new frame is just a duplicate from my previous one, no changes.

The same here, I'll try in the 1.2.19  

For posterity: The tweening script is just for positions (as the main page says.) It clones the pre-frame and then moves it to a spot in-between the positions of the pre-frame and post-frame. You will have to do the rest. If your frames are all kinda in the same position, then you will not see any effect of the tweening.

I'm having this same issue. The file has no groups, nothing appears to be wrong, ive tested it in other files and they work fine, it just does not work in  this one and im unsure why

Was there a solution to this?