You can check if your chain is worn/stretched with one of these…
I would hazard to guess your derailleur hanger is slightly bent but not enough to be easily seen, the cassette is generally worn (though your bike mechanic certainly would’ve caught this I’d think), or maybe you have a tooth or two in one of your cassette gear wheels that is slightly bent. This happened to me and every time I’d shift into that gear, the chain would hit the bent tooth and sometimes immediately be pushed to the next gear or back to the gear I was shifting from if I was pedaling hard. It caused me so many problems on quick and short ups and downs until I just replaced the cassette. But it took a while to find because the tooth was just slightly off and there are a lot of teeth in there.