As we reported, Ben has moved out of their Beverly Hills mansion, and into a home he's renting in Brentwood -- not far from where Jen Garner lives with their 2 kids -- and, although, he and J Lo were both in L.A. last Sunday ... they marked the holiday apart, according to People.

For her part, J Lo certainly had lots of feels on Mother's Day which she shared on social media -- posting about her relationship with her 16-year-old twins Max and Emme, who she had with ex-husband Marc Anthony.

She also dedicated another post to her own mother, Lupe Rodriguez.

Jennifer thanked her mother for "the sacrifices you made of yourself and your dreams to give us the opportunity and strength to live our own" -- and included several pics of herself and Lupe over the years.

We now know Ben was already out of their marital home last weekend, and the estranged couple had not been photographed together in well over a month.

They finally had a brief public reunion on Thursday night, as they both showed up to see Ben's kid, Fin, in a school play. Dad and stepmom brought flowers for the young thespian.

Afterward, Ben drove Jennifer back to Bev Hills, where he dropped her at home -- but then he returned alone to his pad.

They'd both been photographed still wearing their wedding rings earlier this week, and certainly seemed amicable when they were face-to-face -- but we're told they've been separate for at least a couple of weeks now.