Failures in remote-apis-testing with latest buildgrid/buildbox

Context

remote-apis-testing is currently integrating buildgrid 0.0.5 (using the prebaked container) & buildbox 0.0.13, it'd obviously be beneficial to be testing the latest versions. We've tried in the past to quickly bump to buildbox common/worker 0.0.19, whilst keeping buildgrid at 0.0.5 (see here), but at the time they also hung & timed-out in the same way we're now seeing with buildgrid 0.0.7 & buildbox 0.0.21:

https://gitlab.com/remote-apis-testing/remote-apis-testing/-/pipelines/211054260

From a previous investigation into the initial update attempt (after asking @jbonney) setting verbosity on the worker the following output was extracted, indicating 480s polling loop between the worker/server:

https://gist.github.com/tom--pollard/16ddd0d868d19672291fc093c6985804

It's worth noting that when using same version of buildbox common/worker (0.0.21) for recc it seemingly behaves ok with the other server implementations. So there may have been a change in buildbox that is causing the issue with buildgrid in our test pipeline configuration?