I believe the fit width on a popup element is a new feature that was part of the Boost Day from July 16…very likely there is a bug so you might want to submit a bug report about it.
But overall it is a bit confusing as to what is the actual problem and whether or not you are using the fit width on the popup or on a button.
It is also really hard to discern what the settings are for in your screen shots as the top of the element inspector is missing so, not sure if you are showing the same element or which element it is for.
If you screen shot shows the popup with a max width of 100px, that might be your issue, but again, really hard to understand based on the description and screen shots provided.