Around 1-2 weeks ago I sent my bicycle to my dealer because my cassette wasn't moving properly when I examined it by turning my bike upside down and my shifter shifted 2 times instead of 1 and frequently lost tension while riding (felt like the 1st gear for a few seconds). This happened after I rode on rougher terrain and jumped multiple times on a small ramp made by me and some of my friends (it was around 30cm in height).
I went with my bike at my dealer for a bicycle overhaul. They found out that in the gears there was some thin wire (similar to fishing line) stuck in there. They got my bike fixed and I took it home.
It felt much more smooth and all the problems were fixed. Yesterday I took it for my first ride after the overhaul. I fell down after a failed jump attempt at my local skatepark and I'm pretty sure it didn't land on the derailleur side. Fortunately it worked perfectly just like before and it shifted properly. Then I took it offroad where I jumped with it and rode it on a small pumptrack. The same problems started again. Shifts 2 times, loses chain tension, etc.
My bike is a GT Avalanche Comp 2019 (2x9 Deore).
How can I fix it?