I see a lot of people talking about this in the comments of the chapter, and I don't typically like to answer questions about plot/details because I think it's valid for readers to discover, create, and negotiate their own meanings of my work. But, I'll answer this one since I've gotten a lot of questions about it.

Okay, so about the post! Bea's post was from her "diary" account (as seen in the username and caption). From that, you'd be able to know that this is her personal archive of things that happened to her that day. She mentioned in the caption that she bumped into Mason in the grocery and that he's buying a can of Pringles. This post exists as a way to let readers know that Mason is spending the holidays alone. For Maya's POV it also starts a tension in her character arc, but let's talk about Mason first.

From previous context, you'd know that Mason and Bea didn't end on bad terms. At the beginning, Mason mentions feeling partly in debt because he promised to buy Bea the shoes before they broke up which led to him meeting Maya. Later on, he explains that they're still on good terms and that he doesn't have any ill will. It's not in Mason's character to be indifferent towards people he's on good terms with, and you can tell Bea is a bubbly person based on the caption of her diary post. This is why the exes ended up speaking when they bumped into each other in the grocery. Was that right of him? That's up to you to decide and not the story's job to impose. In reality, it's going to differ from person to person. It's completely fine to feel like Mason shouldn't have talked to the ex. And I think it's fair to acknowledge that talking to your ex, in this circumstance, isn't completely irrational.

On to Maya's side of things! Like I said, the post also exists as an integral part to Maya's overall arc. Specifically, her reaction to the post and Mason's comment. In part 119, she hesitated to ask Mason about Bea (ultimately deciding not to in the end). In her diary entry, you can see her feel like she's ready to give up on the relationship. She's not fighting for it. This theme is central to Maya's character—finding strength and fighting for herself/things she cares about. We see a shift in 120-121 after she reads that Mason spent time with Celeste's family. For Mason, this is a small light in the darkness (not spending christmas eve alone). For Maya, it does pretty much the same. Christmas eve allows her to push back a little, and fight for the relationship. By this time, Maya knows there's bigger problems between them than Mason talking to an ex. Their relationship has been secure enough to not leave room for doubt because Mason's integrity was never the question. It was always an internal struggle with Maya's own demons, and she's aware of that!

As for if they've ever talked about it, you can decide based on your own understanding of the characters and the story :) It's okay to feel sad when characters make choices! Stories exist to make you feel. There's no harm in seeing humanity and imperfection in characters. Again, you negotiate your own meaning based on your terms and understanding. No one can take that from you! <33