By default dynmap loads the Survival world, I tried changing it to my spawn world with:

"defaultzoom: 5
defaultworld: Spawn
defaultmap: flat"

However it won't change anything, I've also tried adjusting the zoom with the command but also does nothing. Am I doing something wrong here?

What is your world name?

maybe your world name is wrong.
for example , my server is forge ,and my worlds name are like DIMxxx
you can go to the "world" direction to see the name.

I've checked but have the right name