Thankfully, we no longer have to wait to discover how the pair developed feelings for each other, what went wrong with the duo, and what caused their falling out because Season 6 sheds light on their romance.
Following Jack’s abrupt death, Rebecca and Miguel spend a lot of time together, but their meetings become sparse once Rebecca meets Matt at a speed dating event. Rebecca then invites Matt to Thanksgiving, where Miguel badmouths him to her.
Source: https://www.distractify.com/p/when-did-rebecca-and-miguel-get-together-this-is-us