Feel like that's more the norm than not. Every massive cultural redefining award sweeping TV show has half their cast outright not appearing in anything outside of the show or after it ends.

The other half like a good 2/3rds stay in supporting roles for the rest of their career. Only a few of that portion actually stay relevant with notable roles after "The Show" ends.

The rest just become semi-familiar faces you pretty much only recognize for their work on Breaking Bad/Succession/The Wire/The Office/ Parks n Rec/Game of Thrones/Westworld/Stranger Things/Mad Men/Walking Dead/Abbott Elementary/Sopranos/Crown/House of Cards/Lost/ And the List goes on and on and on... Like by no means irrelevant but not like a name that would make you want to watch something in of itself.

There's the actors who become huge during the show's run, they show up in every other major tentpole movie, 3 Netflix-type movies a year, faces plastered on every luxury brand. But they slowly fizzle out once the next Big Show comes knocking with its own fresh faced unknowns to promote.

Only a tiny fraction of the cast actually breaks into true A-List territory. With projects flowing after their show ends, scooping up awards, collaborating with the prestigious directors, being in A24 and NEON produced movies, just generally becoming an entertainment media mainstay.

tl;dr all that to say you're right, Millie Bobby Brown does seem like she's boutta hit a career limbo with how she's shoehorned into certain movies and nothing that would set her up for roles after Stranger Things ends. Like being included in major tentpole franchises during her stint on Stranger Things is no more a guarantee than Emilia Clarke being in the Terminator reboot, Han Solo movie, or the recent MCU Secret Invasions show. And that's despite my personal opinion that Emilia Clarke is a great actress but seems perpetually thrown into roles with the expectation that she'll Khaleesi it up. Same thing is going to happen to Millie Bobby Brown before the industry picks up a new actress.