If I have 4 Masters A, B, C, D at weight 10, 25, 75, 100 and the /a glyph is drawn and compatible for A and D, how come it doesn’t work to go into B and C and hit re-interpolate to create then from A and D?
Because there is no compatibility yet. When you do that on B, the other 3 are not compatible yet because C is still empty.
Edit: What you can do is move the C master out of the designspace, then reinterpolate B, which now will interpolate only between A and D. Afterwards, move C back into the designspace, and reinterpolate C.
That’s what I did and it worked, I just set one of the other masters outside of the designspace by setting one of the master proportion values on a negative value