And four of the intermediate masters take up positions of masters, so the designspace corners are defined twice. Is that intentional?
Removing them works. But you want to review the designs you put into the brace layers. For instance, in the lowercase s, {800, 60} is not as bold as to be expected at wght=800.
I have tried those double designspace corners, because it was even worse before I had them (but then I also had 4 more masters). And also I thought that this is what Georg meant by fully cutting through the designspace. But maybe I misunderstood.
Also, you are right, the intermediate layer instances behave weirdly. And the designs are not correct. It seems, the intermediate layers are based on other intermediate layers and not on the masters. When I re-interpolate all the layers multiple times , they keep changing.
Then I do not know what your design intention is. Is wght=800 supposed to look bold? If yes, why do you put nonbold drawings on wght=800? See the intermediate master on 800,60.
Edit: Ah wait a minute. Just saw that you sent a new file. Will take a look