If you want to mount both of these to assets
:
mysite/assets/
mysite/foo/
You need to mount both:
module:
mounts:
- source: assets
target: assets
- source: foo
target: assets
https://gohugo.io/hugo-modules/configuration/#module-config-mounts
When you add a mount, the default mount for the concerned target root is ignored: be sure to explicitly add it.