The actress dished about the awkward moment from back in April at CinemaCon -- where she was showcasing the trailer for her new flick, "Don't Worry Darling" -- as a random person walked right up to her in the middle of her presentation and laid a manila envelope at her feet.

As it turned out, they were custody docs from Jason ... who was trying to settle a legal matter about their shared children and where they should live. In the end, that didn't go his way.

Olivia had already telegraphed she was utterly outraged by what she considered a deliberate sabotaging at the hands of Jason -- even though he's denied knowing that's how she'd get the docs -- and now, she's going even further in Variety ... calling the event "vicious."

Olivia says this happened at her "workplace," which she finds astoundingly inappropriate. She also sees it as scary, considering the amount of hurdles that the process server must've gone through to get in that day ... which, in her mind, means it was planned well in advance.

She also had this to say ... "I hated that this nastiness distracted from the work of so many different people and the studio that I was up there representing. To try to sabotage that was really vicious. But I had a job to do; I’m not easily distracted."

Olivia takes another personal jab, adding ... "But, you know, sadly, it was not something that was entirely surprising to me. I mean, there’s a reason I left that relationship." Oof!

As you know ... Jason and Olivia were together from 2011 until 2020, when they surprisingly split and shocked Hollywood. Not long after, she started publicly dating Harry Styles -- who also stars in her new movie -- and they're still going strong.

It's interesting ... both Olivia and her current bf graced different magazine covers this week, but it seems HS was a bit more chatty about their relationship ... opening up about how some have gone on the attack against her due to her affiliation to him, which he regrets.