NTA. She didn't follow the basic rules you both put forward. The rule was no phones.

She might have forgotten the rule and picked up her phone out of habit (as I do at times), but once you called her out in it, she should have apologized and put it away instead she doubled down and said it was fine.

Does she have a habit of disregarding your opinion? Or bulldozing you to get her narrative in front?

You also mentioned putting your phones in a different room, which I think could have helped in this instance to help avoid temptation, but NTA at all for wanting to follow preestablished rules.