Hi, I have a new Cannondale Trail 5 mountain bike with Altus shifters and Alivio derailer. I've put about 12 hours on the bike, and when I shift from 4th gear to 3rd gear going uphill, the chain moves immediately to the correct gear, and I can feel the pedaling get easier, but 4-20 cranks afterward, the chain slips on the gear for a half-crank or so, then drops back onto the gear, and stays where it's supposed to be until the next time I shift from 4th gear to 3rd gear. This occurs almost immediately under heavy load; It can take up to 20 cranks if under very light load. (like if I don't really need to be in 3rd gear, but downshifting a little early in anticipation of needing to be in 3rd)

Some other information / observations:

I weigh 200 lbs.

I cannot reproduce this behavior on flat pavement or even on a decent sized hill on pavement - It only happens going uphill on a trail, and it happens 95% of the time or more

I have not experienced a single shifting malfunction shifting into or out of any other gear

This happens regardless of which front ring I am using

LBS where I purchased the bike suggest user error. An employee, about 150 lbs tested the bike on relatively flat pavement and was not able to reproduce

I am not an experienced biker but logic would dictate that if I have 0% issues in all other gears and problems every time I shift from 4th to 3rd, it's not user error

Any opinions on what could be causing this?