ERG mode can feel very counter-intuitive at first. When the trainer is telling you to pedal at a particular power level, it adjusts the resistance to your cadence, so that you produce the correct power.
The counter-intuitive bit is that the faster you pedal, the less resistance you’ll feel (opposite to normal cycling), because more rpm needs less resistance to produce the correct power. And the slower you pedal, the greater resistance.
So to allow it to work, you need to hold a relatively steady cadence. Depending on your trainer model there might be a slight delay as it adjusts the resistance.
So if you keep pedalling faster and faster, the trainer will keep reducing resistance and you’ll end up with the scenario that you describe.
Use the small ring, middle of the cassette, settle on a cadence and give the trainer a few seconds to adjust resistance, and it should work.