bake cache misses with multiple targets

I was hoping to speed up builds by using the parallelism provided by bake.

Therfore i'm running buildx bake -f docker-bake.hcl

group "default" {
	targets = [ "s1","s2","s3","s4","s5","s6","s7","s8","s9","s10","s11","s12","s13","s14","s15","s16","s17","s18","s19","s20","s21"]
}
target "s1" {
	context = "./s1"
}
target "s2" {
	context = "./s2"
}
target "s3" {
	context = "./s3"
}
...
target "s21" {
	context = "./s21"
}

but nearly all of the time one (or more) of the images gets rebuilt, even if they should hit the cache.

 => CACHED [s1 build-app.web 2/7] WORKDIR /app                                                                                                                                                                0.0s
 => CACHED [s1 build-app.web 3/7] COPY package*.json /app/                                                                                                                                                    0.0s
 => CACHED [s1 build-app.web 4/7] RUN npm ci                                                                                                                                                                  0.0s
 => CACHED [s1 build-app.web 5/7] COPY ./*.json /app/                                                                                                                                                         0.0s
 => CACHED [s1 build-app.web 6/7] COPY ./src /app/src                                                                                                                                                         0.0s
 => [s19 build 6/6] RUN npm run build                                                                                                                                                                         2.8s

If i specify single targets, the cache is used as i would expect:
for i in {1..21}; do buildx bake -f docker-bake.hcl s$i; done

What if found out so far:

• The more default targets i have, the less likely the cache is used for some of them.
• When i try to reproduce this with trivial Dockerfiles containing only a single RUN the problem doesn't occure

Are there any ideas why this is happening or how i could investigate this further?

Start by reproducing the issue with the supplied docker-bake.hcl and the multi-target buildx bake command, then compare it with the sequential single-target commands. Reduce the example from the 21 targets and contrast it with the trivial Dockerfiles that do not reproduce the problem. Done means identifying why cache use differs for parallel targets and documenting a reliable reproduction.