Guest WLAN on OpenWrt master or trunk

I want to create a guest WLAN by following this guide
The operation succeeds correctly on OpenWrt 19.07.2 and on any other stable version. If I perform the same procedure on a Developer snapshot or if I build on master branch, the guest WLAN does not work. Whether you change the configuration in the local build, or leave everything default (except for LuCI), the guest WLAN still does not work. The problem is in the guest interface, which, instead of returning the IP address, displays this error:

It looks like something connected to the DHCP server. If I set the guest interface DHCP server to force, the guest interface works regularly until I restart. Then again it doesn't work.
The error is reproducible for anyone on any device as long as you use a developer snapshot or build on master. Applies to both 2.4 and 5GHz. What can it be? Something that needs to be configured/built?