Not listed as author despite doing statistical work
Talk to the PI who said you would be an author, or your boss, whoever you are more comfortable talking to first. Best to assume it was an honest mistake (there's really no cost to adding another author in medical sciences). There's a bit of gray area on when statistical support is included/not included in authorship in medical science.
Let them know you expected to be an author (and remind them of your previous conversation), but that you found a preprint that didn't have you listed.
A preprint is just a preprint; hopefully you can still be included on a submission of the paper to an actual journal.