I have 3 submission posted on the same date. Two of them were upgraded but somehow only the non-upgraded is “in voting” while the upgraded ones are “in queue”.
It would makes sense to be the opposite. The submissions are in the same area.
There is evidence that each submission is reviewed by the ML system in the first 24 hours, after which it gets held for a random period between 1-7 days before being able to going into voting.
Upgrading a submission makes no difference to this. Upgrades do not make a submission enter voting faster, but may make the voting take less time because it opens the submission to a wider pool of reviewers.
The location can also make a difference, if the submissions are in different areas. Each area is limited in how many submissions can be in voting at any one time, so a submission in a busy area will take longer to go into voting than one in a quiet area.