Interpolation problem in variable font

Hi all,
I’m developing a variable typeface built around 3 axis:
weight (300-900)
width (100-200)
serif (1,10)

I started working with 4 masters:
Light (300,100,1)
Black (900,100,1)
LightSerif (300,100,10)
LightExp (300,200,1)
And everything worked pretty much as expected.

I then added a 5th master to have control on the stems’ thickness in heavy weights for the sans typeface
BlackSerif (900,100,10)

As you can see from the video below, whenever all 3 axis work at the same time, the result is quite messy, and it breaks completely on the endpoint of the interpolation bar.

Looking at the position of the masters in the design space, it seems quite extreme.
Why do I get values such as -100% black, -100% LightSerif and 200% BlackSerif? What am I missing?

All instances are within the design space but there is something funny going on and I can’t figure out what is happening.
Hope it’s not too much of a newbie question. Cheers!

What is your origin?

You most likely needs some more masters to make this stable. And try testing in FontGoggles or a browser based font tester.

the origin is the Light (300,100,1).

I tested on Samsa and Dinamo’s font Gauntlet as well and the design didn’t break so much, but seeing that result and the massive values (+200%, -100%, which i still can’t figure how tbh) I was worried I might have overlooked something when setting up the masters/axis/values.

Can you send me the file?

File sent

As far as I can tell it works as expected. To get rid of the outline issues, you need to add some more masters. But as it works in most glyphs, you can try to add some brace layers in those glyphs.

Super. Many thanks!