Hi Blindfoldjump - basically the way inset works can make trying to inset a smooth set of faces to be difficult - that's because the main way that Inset tries to do things is by keeping one face in place and then doing offsets on the surrounding faces. When all the pieces meet up smoothly that method does not work - when it's just a planar face in the middle of a bunch of smoothly connected faces it will try to use a different method but when you are trying to do multiple pieces all meeting smoothly it's a difficult case for it.

Inset tends to work better when used on a more structural entire face like a face of a cube that's bordered by sharp boundaries on all sides of it, it has difficulties on panels that are carved out of a larger shape such that the panel is all smooth to its neighboring faces.

What you might want to use on a narrow smooth strip like that is to use Construct > Shell to thicken it into a solid, then if you have the main solid all still in place rather than interrupted by a hole you can use boolean difference to cut the thickened piece from the main body, otherwise if you have cut an open hole you would delete the top faces of the thickened object and then use Edit > Join to glue the 2 pieces with unjoined edges together into a finished joined piece.

- Michael