Bed not detected right in slicing/positioning

[AbeFM 29]

I've seen this in a bunch of threads so far, parts not dropping to the bed correctly. Often, I'll see something like this, where the brim goes right through the model:

 

 

The problem here is the part Definitely doesn't have a gap there. After seeing the weird brim going under a flat surface, I assumed it was some rounding error... so I dropped the part 10 microns into the bed, with the same result. So then I dropped it halfway into the bed, so it's completely solid above and below the bed, and get this strange behavior.

 

Getting models to load properly into CURA is one of the big challenges, and oftentimes pushes me to another slicer because 20 minutes of tweaking to just get it to lie flat on the bed offsets any other gains the slicer may offer.

 

 

This is the refusal to drop to bed. In some views (not sure why here) you see a large bounding box, which gets respected as the edge of the object for dropping to bed purposes, but not for printing - supports will fill in the area.

 

 

If there is some suggestion on how to position things without zooming, manually placing (after turning off drop-to-bed), slicing, looking for support or missing structures, re positioning, slicing again, and repeating till it's within 20 microns.....  It's just not a great workflow. It happens with models I make, models I download, maybe 1/4 of the time in total. It has been this way since 3.0.