I just ran into this problem, and wrote up a way to work around it (without having to remove the l3-interface, reboot, and re-add the interface to change the netmask) in this thread: https://forums.juniper.net/t5/Ethernet-Switching/Overlapping-subnet-is-configured-under-irb-ERROR/m-p/310863.
Hopefully this helps anyone else who runs into this quirk, and finds these threads in the future!