When you track conversions by importing goals from GA, I believe GA only attributes the conversion to the last click source.
So if you come from Ad -> Organic and convert at the organic level, GA won’t attribute the goal to Google Ads but rather Organic. No conversions in this case will be logged by Google Ads.
On the other hand, if you set up conversions by GTM/hard coded, as long as the user clicked the ad and converted (even if they converted via another source), Google Ads will log the conversion.
Hope this helps. Maybe someone with more knowledge can chime in.