racket/unit behavior "bleeds through" to different file

I just found a fun little bug that appears to go back to at least racket v7.5. It appears that under some circumstances, opening a file not written in the racket/unit language can trigger a warning about missing imports, along with a clearly bogus highlight.

Steps to reproduce:

1. delete your proferences. On MacOS Catalina, go to ~/Library/Preferences, delete org.racket-lang.*
2. Open DrRacket.
3. use flower-l, flower-r to choose "The Racket Language".
4. Enter this program:

#lang scheme/signature

5. Run.
   5a) Save the new file to /tmp/zz.rkt.
6. Open a fresh tab.
7. open the stepper/private/view-controller.rkt file (easiest using cmd-shift-o).
8. Find the "import" clause on line 37, comment it out. (See error at bottom.)
9. Save the file.
10. Open the file "xml-sig.rkt" in the same directory.
11. The interactions window should now show the error "#%module-begin: missing an `import' clause in: (#%module-begin)", and the red highlight should highlight part of scheme/signature. (See screenshot.) This state seems clearly wrong to me because the scheme/signature language should not require an import clause, and DrRacket should never highlight just part of an identifier.

Reproduce the issue in DrRacket using the provided preference-reset and file-opening steps, starting with stepper/private/view-controller.rkt and then xml-sig.rkt. Inspect the interaction-window error and red highlight after commenting out the import clause. Done means opening xml-sig.rkt no longer inherits the racket/unit behavior, reports a missing import for scheme/signature, or highlights only part of the identifier.