There is a bug with grouping repeated notes to create measured tremolos.
In the manual on page 1196 you can see this:
The result we get:
Did you post this previously, then delete it? (I replied to that one, but the post had vanished before my reply got “approved” 
)
Anyways, yes, I concur, I get the same result. Workaround,…after performing the Repeat operation, just Hide the 2nd and 4th sixteenth notes.
I did report this and you confirmed, but it was for the previous version
I copy here your complete workaround for those who are interested:
“Hide the unwanted notes manually, then do Group/Repeat as 16ths, or just do Group/Repeat, then Hide the 2nd and 4th 16th notes and then make sure Display Quantize is set to 8ths!” (vic_france)
Nice workaround which can become laborious in a passage with dozens or hundreds of repeated notes, which is by the way the main use of such a function.
Thanks again for confirming Vic.