reprotest fails when RELEASE is set to experimental

I triggered a salsa CI pipeline from a branch where gitlab-ci.yml has:

variables:
  RELEASE: 'experimental'

all steps pass except reprotest: https://salsa.debian.org/js-team/node-yarnpkg/-/jobs/470201

from the log:

$ apt-get update
Get:1 http://deb.debian.org/debian sid InRelease [142 kB]
Get:2 http://deb.debian.org/debian sid/main amd64 Packages [8194 kB]
Fetched 8336 kB in 2s (4455 kB/s)
Reading package lists...

whereas the build step from the same pipeline which works fine has

780fb5b7e12b5e4869459c5f21d73aad71abd53b1d9ade6ce9d4670a898e5a91 apt-get update
Get:1 http://deb.debian.org/debian unstable InRelease [142 kB]
Get:2 http://deb.debian.org/debian sid InRelease [142 kB]
Get:3 http://deb.debian.org/debian experimental InRelease [72.2 kB]
Get:4 http://deb.debian.org/debian unstable/main amd64 Packages [8194 kB]
Get:5 http://deb.debian.org/debian sid/main Sources [8463 kB]
Get:6 http://deb.debian.org/debian experimental/main amd64 Packages [377 kB]
Fetched 17.4 MB in 4s (4769 kB/s)
Reading package lists...

perhaps the reprotest step is not picking up the release variable ?