I’m torn on this. I understand the expectation of not having automatic backups in read-only mode. You usually create manual backups when the site is readonly and don’t rely on automatic ones.
But, and this is a big but, disabling automatic backups increases the chance of data loss. Should an automatic backup be created if there is no backup yet? How can we ensure that the site is really readonly all the time? The backup process might be unlucky and always run at a time when the site is readonly for whatever reason. Users could have created content in the time when it wasn’t readonly…
Solving this would complicate things. And ignoring those problems creates risk for site owners. Discourse tries to have safe defaults, so I think the best and only solution is to manually disable backups before permanently switching into readonly mode.
So, I’d say this isn’t a bug.