OK I’ve worked out why this was happening: the 5.23 builds were made on a different server from previous builds, and that server was not setting the OTA_URL environment variable correctly. The problem will not arise with the 5,24 builds because they were build on a server where the the OTA_URL environment variable is set correctly.
See this issue for more details
EDIT:
PS You won’t see any OTA updates until you manually install a version that is not 5.23.