The mouse buffer menu has a few customizable variables, a couple of which are
mouse-buffer-menu-mode-groups. The default value for
4, so setting it to a higher value seems appropriate in this case -- e.g., a number high enough that grouping will not occur.
To read the doc-string for a variable, type
To customize the variable, type
The seemingly randomness of grouping (sometimes yes, sometimes no) indicates a bug. Someone who is motivated may wish to submit a bug report in relation thereto.