'Stranger Things' Actress Cut From Finale Was Warned About Recent Comments

A "Stranger Things" actress left out of the finale season has made headlines after speculating about her exclusion ... and now TMZ has learned she's been told it'd be wise to pipe down.

Here's the deal ... Jennifer Marshall, who played Max's mom, remarked on fans questioning why her character went MIA from the series' final season ... the actress suggested producers didn't want to roll the dice since she was fighting stage 3 melanoma IRL.

Jennifer tells TMZ … she had no idea her comments would go viral -- and she wants to clarify that she wasn't knocking producers Shawn Levy or the Duffer brothers for not being invited back ... even if it sure hurt like hell to be ghosted.

And the real kicker ... since sharing speculation about her omission ... Jennifer has heard from one cast member and a couple of crew members that her comments could get her blacklisted in the biz. She doesn’t care.

JM tells us she kept her cancer battle to herself as long as she could ... until rumors began to spread on set during Season 4 about her using drugs.

Jennifer says she then revealed her health struggles to the brass ... and she says there seemed to be no issue or concern about her abilities with the explanation. Ultimately, Jennifer tells us she doesn't really know why she wasn't brought back … and points out it made for a plot hole fans definitely noticed.

The entire situation went from bad to worse because, without any of the new "Stranger Things" work, she didn't meet the SAG threshold for health insurance ... which she desperately needed.

It's one of the stranger things about the final season ... and it seems Marshall isn't worried about letting sleeping demodogs lie.