This is because caustics are currently not rendered where they are not "needed". So if some area with caustics is not visible in the beauty pass, it is not rendered, and this makes rendering faster. This is something like adaptivity for the caustics.
While this is perfectly fine for most beauty renders, I can perfectly understand that the caustics sometimes need to be extracted for some post-adjustments, and then it fails.
I will find out what are the exact plans regarding this, and whether there is some way to disable this specific feature under the hood.
We already have it reported by other users:
(Internal ID=460381847)