it seems that for a while either none or the same list of commits keep being added to release info on the nightly builds (https://github.com/vyos/vyos-nightly-build/releases). bit of a shame since it was a nice place to keep track of what was in which nightly build.
I have quickly looked over the workflow and ran it in a fork for as far as I could (fails on lack of secrets) but the PREVIOUS_SUCCESS_BUILD_TIMESTAMP seems to be correct, as do the usage of it later on to get the new commits on both vyos-1x and vyos-build. I cant see any reason for it not working without being able to look in the full logs of the workflow running, hopefully someone can.