yep, that does indeed seem to be the case... if there's anything obstructing the top of the slide scanning area it goes stripey or even refuses to scan at all... annnnoying... sideways or not reaching the top seems OK but of course that's not much help when it comes to scanning decently long segments from the middle of a reel...
Anyway, got sidetracked scanning some of the heap of tiny clippings + outtakes from my films which I've obsessively horded over the years !
Some pretty mysterious images in there!

Concrete evidence of moon landing hoax! 8O Secretly filmed on covert 8mm Svema by Russian psi-spys!!

A monster with a monster for a foot; the latter dividing into a third monster! Plus, a finger.

TERMINATOR! (Nighttime K40, good job I had a XL camera all those years without really understanding what that meant!)
The annoyance of stripiness aside, I'm surprised how well these tiny frames scan... I've been doing them at pretty high resolution and tho the scanner isn't super sharp (and may be simply interpolating when I set it to very high rez for all I know?) I'm pretty pleased with some of the results... with a bit of sharpening and levels correcting I should be able to make some nice images. I'll scan a load some time and put up a page on my site of stills from my various ancient works, been meaning to do that for ages...