This has been a one-off change, where the default mode was changed from “all photos” to “manual selection” (this is actually the reason for the catalog format update in 1.0.0-rc.49). Setting it back to “all photos” explicitly should work as expected, though.
The background for this change was that the previous default of “all photos” makes is much more error prone to add new external disks or NAS folders to the library. With “all photos” on by default that would mean that all files from the external disk would immediately get copied over to the main library instance after the initial clone, then likely leading to a full system disk. With “manual selection” you’d instead get a warning about insufficient synchronization settings at some point, giving you the chance to make an explicit decision.