get step can't find pinned digest

[cjnosal]

I have a resource watching for semver_constraint tag (e.g. >=1.17), and some of the underlying tags are floating.
e.g. fixed tags 1.17.3-photon-r1 and 1.17.3-photon-r2 (which are ignored by the resource due to #307) and floating tag 1.17.3.

I pinned a version of the resource (e.g. 1.17.3@sha256:12345). Then sometime later, a new revision of the image comes out (1.17.3-photon-r3@sha256:45678) and 1.17.3 is retagged.

Expected behaviour: the pinned version has a digest, which still exists. Rerunning a job should use the pinned digest.
Actual behaviour: the job is stuck pending because the pinned version can't be found.