It seems that there is a bug with grouping repeated notes to create measured tremolos.
In the manual on page 1190 you can see this:
Results that I get:
Can anyone confirm please?
Same here (even in earlier versions).
The only way I was able to get it like in the pdf, was to Hide the unwanted notes manually, then do Group/Repeat as 16ths, and then make sure my Display Quantize was set to 8ths!
… or… just do Group/Repeat, then Hide the 2nd and 4th 16th notes 
Thanks for confirming (and for the workarounds) Vic.
It’s too bad repeat functions (including Brillenbass, but it’s another story) don’t work as they should.