Dr. Racket does not combine "Combining Macron Below" with previous character when rendering unicode

On macOS, in racket, if I run
(bytes->string/utf-8 (bytes #x77 #xcc #xb1))

I get:
"w̱"
(w with an underline below it), but in Dr. Racket I get a w with an underline next to it, as though they were separate characters.

See:
https://en.wikipedia.org/wiki/Macron_below

For context: I noticed this while representing the "Squamish" in its native orthography:
(define B
(bytes #x53 #xe1 #xb8 #xb5 #x77 #x78 #xcc
#xb1 #x77 #xc3 #xba #x37 #x6d #x65 #x73 #x68))

(bytes->string/utf-8 B)
"Sḵwx̱wú7mesh"

In Dr. Racket, the k is properly rendered with the underscore (to be fair this is a built-in unicode character)

But the x renders without the overlap.

Interestingly enough, running string->length on the above gives "12", but I'm guessing that this is the appropriate answer if "combining diacritical marks" should count toward length

Start by reproducing the issue in Dr. Racket with bytes->string/utf-8 using the combining macron-below example, then compare it with the built-in Squamish character rendering. Check the rendering path reached from these examples and verify that the combining mark is visually attached to the preceding character when done.