Having done the same change i didn’t experience anything in those regards with my 13v3. Not having a proper idea, i remember having read something 'bout screenflickering here on the forum. A search might be helpful. It COULD at least be a coincidence not directly connected to your change.
If you’d change back to the old card, you could at least find out, wether it’s connected to the new card.
On detaching/attaching the micro-connectors make sure to drag/push them absolutely straight up without any angle. Otherwise they’re prone to break (i experienced exactly that - no fun i can tell you).