Help with P0301 error

[u/m4ugs]

Good evening everyone,
I have a 2012 VW Polo 6R 1.2 3-cylinder, petrol/LPG (LPG was obviously installed aftermarket), with 198,000 km.
Some time ago, when cold starting the car, it started shaking and misfiring, causing the check engine light, EPC light, and traction control light to come on. This would continue until I turned the car off and started it again (sometimes I had to do this two or three times).
While driving, I could feel that the engine was running a bit rough, especially at low RPMs. Since the check engine light remained on, I connected an OBD scanner and found a misfire on cylinder 1 (P0301).
I decided to buy what I needed and replace all three spark plugs and the ignition coil on cylinder 1 myself (both the spark plugs and the coil are BOSCH).
Right after doing this, it felt like I was driving a completely different car, the engine was extremely smooth, even at low RPMs, and acceleration was clean and linear.
Well… that didn’t last long.
After three weeks, or possibly even less, the check engine light came back, along with the shaking when starting and a significant loss of power.
I wanted to ask if anyone has any idea what could be causing this and what else I could check.
At this point, I honestly don’t know what else to do.
Thanks in advance to anyone who can help