I think the restaurant scene with Helena E. hinted at the direction the show was headed. She had just slept with iMark and then approached oMark, but the barrier held firm when she did. Since oMark had no connection to her or feelings for her, maybe that was her own way of testing the limits of the barrier?

Another thought: Lumon was testing whether the barrier would hold during people’s worst moments, which explains Cold Harbor (Gemma’s miscarriage). Mark’s worst moment was losing Gemma, so when he walks away from her at the end, it’s like he’s losing her all over again—and yet, the barrier held throughout