Animate CC: Animation looping to first frame but looping to the first and then stopping

I'm creating HTML banners in Animate CC. I've created them before and never encountered the following problem...

My animation is looping through to the first frame rather than stopping on the last frame. Loop timeline is unchecked in publish settings, and I've tried adding this.stop() and stop(); to the actions but it's not working - please can someone advise if there's anything else I can try?

Thanks,

Beth