As we reported, Idris revealed Monday he tested positive for COVID-19, but actually went into isolation on March 13 when he discovered he'd been in contact with someone who had the virus -- possibly Justin Trudeau's wife -- during a March 4 event in London.

The bigger problem, though -- the actor spent the next several days after that event in Sante Fe, New Mexico rehearsing with his costars and crew members for the upcoming Netflix flick, "The Harder They Fall."

So, if Idris really did contract it in London ... he would've unwittingly exposed some people on set. Sources close to production tell TMZ ... filming was halted last Friday, and the crew was told it was due to growing fears over the pandemic. We're told there was no mention Idris was getting tested.

As for why they're concerned -- we're told Idris had some direct contact with members of the stunt team during rehearsals ... part of which took place in a small cabin.

Yet, our sources say nobody was informed them Idris tested positive until the actor revealed it himself on March 16.

That same day, we're told some of the crew traveled home -- possibly exposing even more people -- but other crew members chose to stay and self-quarantine ... and they're still there. Idris is also still self-quarantining at a house on location, while the crew remains at a hotel in Santa Fe.

No one's peeved about the housing situation ... it's more the lack of communication about potential risks following the news about Idris' positive test.

We're told, as of Friday, no one who worked on the film has been tested, and many people feel more should be done to protect them or give them some peace of mind.

Of course, the shortage of COVID-19 tests has been a major issue nationwide, but if that's the case ... our sources say production or higher-ups at Netflix should be conveying that, but so far they've been silent.