I don't know if this right but
in U13 i have big problem - can't select / cut / assign materials for elements that are hidden, that are behind other objects in illustration mode
For example i have a wall behind glass window. I want to attach material to the wall - but it's impossible in illustration mode - i can't select objects behind other
See also my tread
Maybe is something in preferences / setup that prevent illustration mode selection ? Maybe some select - view depth in illustration mode ?
and short video - see i can't select > https://youtu.be/JAlfZK4Xo2Q
As Nilesh said : " Preferences > Input > Locate Interiors is set to 'Visible' "
This works if someone have a problem too.