Struggling with a caching substitute server

[Pjotr Prins]

I am running client and server in exactly the same checked out guix
repository. Server:

  ./pre-inst-env guix publish -u guix-publisher
    publishing /gnu/store on 0.0.0.0, port 8080

Client on the same server

  ./pre-inst-env guix package -i hello --dry-run
    The following derivations would be built:
     /gnu/store/ipsmyhrsfnfvyb096fpiscd7rn3d9557-profile.drv
     /gnu/store/l6z8ln2r4x9g2y6yy62ly3f8jx2k1rxy-ca-certificate-bundle.drv
     /gnu/store/v79w8yj766ryqnc6i1k8ry6wi8vfhy3f-info-dir.drv
  The following file would be downloaded:
     /gnu/store/zby49aqfbd9w9br4l52mvb3y6f9vfv22-hello-2.10

After installing the package it is in 
/gnu/store/zby49aqfbd9w9br4l52mvb3y6f9vfv22-hello-2.10


Now from a different remote machine using my substitute caching server

  ./pre-inst-env guix package -i hello 
--substitute-urls="http://guix.myserver.org"; --dry-run

keeps on wanting to build the thing from scratch.

While from that same machine

   curl http://guix.myserver.org:8080/nix-cache-info
     StoreDir: /gnu/store
     WantMassQuery: 0

appears to work fine.

The response for both queries is:

Query from main server:

GET /nix-cache-info
GET /xgm6wxkja6llvkz8jck95i6cmh3c93yh.narinfo
GET /w6im8ck36hpjyh9zjmz9mr2vkppfhzid.narinfo
GET /9wq9z5jq3g691f836lhi0633g26lfl8n.narinfo

Query from remote

GET /nix-cache-info
GET /x2lhjhbxd5dqmis350gkc352v9z1pr8r.narinfo
GET /jkxa2ga4x0gqvx6x790anpwhnm7d278w.narinfo
GET /ic8sabq35jisj5wmi860d85i09qghq1x.narinfo

which suggests the files it wants to fetch are not the same, though the 
starting point 
(the checked out guix dir) should be exactly the same. The last commit is the 
same.

When I use hydra, no packages from source are built and the package
installed in exactly the same path
/gnu/store/zby49aqfbd9w9br4l52mvb3y6f9vfv22-hello-2.10A

This is my reproducible build environment. I would like tho have the
caching substitute server do the right thing. What am I missing?

Pj.