A missing ground can make things behave oddly as the current will find a path to ground through another bulb if it can.

If you are 100% sure the ground is solid and on the correct terminal, is it possible the new housing was made incorrectly? I wonder if they only had one as a sample and when mirroring it for the other side got the order of the terminals wrong. If so and the vehicle side plug isn’t molded, you can pop it open to rearrange it to match the housing.

Michael