Dragonite & Gardevoir...

Rounding issues. Instead of canceling each other out the damage dealt comes out at 0.999%. In effect it's the same damage but still technically less than neutral, hence the message.

When a weakness and a strength cancel, the decimal calculation leaves it something like 99.97% effective. This still triggers the NVE text even though it is neutral damage in the vast majority of cases. (Theoretically, the difference could cause you to drop under a break point on rare occasion.)

The above responses are correct, but really it is a bug because Niantic didn’t properly account for it. It should be messaged as nuetral.

The above responses are correct and whoever downvoted them is wrong. Some more details:

Supereffective = 1,4x damage modifier
Not very effective = 0,714, which is truncated from 1/1,4 = 0,714285...

Because Niantic rounds the NVE modifier, the result of multiplying SE with NVE = 0,9996, which is less than 1 and triggers the NVE message.