I'm making a font named 'LS Ultra Grotesk' using Fontlab 7.
It's a Multiple Masters, with 6 weights. Here's from Font Info > Instances.
I'm exporting the font to Web PS (otf) using these Instances.
If I install these fonts, Windows doesn't recognize them as a family.

Windows wrongly reads the regular weight as family: LS Grotesk with style: Ultra.
What should I do?
Comments
As a test, try changing the family name to, say, “LS Ultre Grotesk” and see if that behaves as expected, or exhibits the same unexpected behavior.
Should I try using numbers?
WPF font selection model - Microsoft (windows.net)
I guess I should look for another family name?
- a font caching problem
- you have changed some name table entries, but not all the relevant ones
If I am not wrong, you should consider LS Ultra Grotesk into LS Ultra Grotesk Bold.