New water supply lines continue to leak at toilet tank connection
To answer your question could the plastic threaded inlet be the problem , possibly. That pipe could be cracked or the seal in between the fill valve and the tank could be leaking. This is a tough location to get a good look. Normally I find them over tightened but it sounds like you have been careful.
I might turn the water off drain the tank and hold the float up,
no water dripping turn the water on see if it starts dripping float still up, this would indicate a crack in the plastic and a need to replace the fill valve.
If it doesn’t drip but starts once a few inches Of water are in the tank drain again pull the fill valve and use a little silicone calking on the seal tighten it back in place let it sit for 15 minutes or longer to set, then try filling again it may have been a leak that you just sealed with the silicone.
At this point if it still drips look for hair line cracks in the tank itself.
Those are the things I normally check quite often I have found cracked plastic fittings after being over tightened.