Yes! I see a pattern now.
If I fill out ALL the slots with a music choice. I get what appears to be good random selections. If I remove some choices and leave them blank like 7-9 then it seems to play from slot 0 much more often. It appears that the occurrence of playing music 0 matches with the number of choices I removed. So it's like the random number generator always picks from 0-9 and if the choice came up from 7-9 it will default back to choice 0. If I remove more choices the more often music option 0 plays. So with only 2 music options it plays from 0 about 90% of the time. As I add choices the frequency of music option 0 playing becomes less.
I filled out choice 0-4 and left 5-9 blank. I was getting choice 0 around 60% and any of the 1-4 options 40%
Bug!
There I think you know what to look for now.
(I found it easy to test by uploading the prompt set recordings of digits 0 through 9 to their respective slots in music on hold so I knew which slot the hold music was playing from)
Now to get my 2 music choices to play 50/50 I filled out 0-4 all with the same song. Choices 5-9 all with the other song. I'm getting proper 50/50 random hold music but this is only a workaround to the random number selection bug behaviour.