[Support requirements not recalculated after rotating object]

I am using Cura 3.6.0.  I'm not sure what screenshots would help, but I've included one.  The screen in Cura looks right, but the entire area of the box is full of supports even where they are supporting anything.

 

[Support requirements not recalculated after rotating object]

I have a 5 sided cube (open on the bottom).  I load the STL into Cura and it is sitting with right-side up.  In that position, Cura says supports are necessary for the entire top.  That makes sense since it is like a "roof".  However, I rotate the image so that it is upside down to print.  Cura still wants to put supports in the same place.  If I look at the bottom of the object, it is all red (meaning needs supports) and when I print it, the entire object is filled with supports as if it needed them to support the top.  How can I get Cura to recalculate where it wants to put supports after I rotate or move the object?