Gears switch automatically on smart trainer I need some help. I am a newbie when it comes to bikes, so feel free to ask me anything needed.
I bought this bike half a year ago.
I recently bought a smart trainer (Tacx Flux S Smart), and innocently bought an 11-speed cassette, which I then realised it wouldn’t work properly with the bike above (it has a 9-speed cassette, 9-speed derailleur, etc).
I returned the cassette and bought a “SHIMANO CASSETTE HG400 9 speed 11-34”. I mentioned that my bike has a 9 speed 11-42 cassette, but was told 11-34 should be fine.
I just mounted it on the smart trainer, and while doing the calibration, some gears shift automatically when accelerating/reducing speed , specially in the smallest cogs, so I am trying to understand why and these are a few of my hypothesis:

Do I need to put some lube on the chain? Looks like it doesn’t need, but can this be the problem?
Do I need to adjust something in my bicycle’s gears?
If there is a compatibility problem, is there a cassette that you know would be for sure compatible with my bike?
If not, what sort of adjustment can I make? Do I need to get a derailleur / everything of the same brand?

Any help you can give is welcome as I am a bit clueless on the problem.
Thanks in advance!
First, this is not a between-brands compatibility issue. Microshift products are generally compatible with Shimano. In fact, in 11 speed and below groupsets, Shimano, SRAM, Microshift and others all use the same standards for cassette sprocket spacing and chain width, and there is a general cross-compatibility.
Freehub bodies for 11 speed road groupset cassettes are slightly longer than those for 10 speed or less. 8, 9 or 10 speed cassettes need to be installed with a spacer behind them. Check that you used a spacer when you installed the 9 speed cassette on the trainer.
Also, on different manufacturer and model hubs (both on wheels and trainers), there is some variation of the exact lateral position of the free-hub body that holds the cassette. That means that if you swap wheels or put a bike on a trainer the adjustment of the rear derailleur will usually need to be changed.
Run through a rear derailleur adjustment process with the bike in the trainer to properly set the upper and lower limits, and the sprocket indexing. Park Tool Repair Help has very good step-by step instructions..