As of writing this post, when you have a custom package selection and request a build nothing happens (no screen feedback, no errors, no acknowledgement of the request). This seems to be the case for the latest stable and RC builds from what I have tested. I have tried three different browsers to counter a potential caching issue. The issue also persists for both the https://firmware-selector.openwrt.org/ version and the https://openwrt.github.io/firmware-selector-openwrt-org version of the firmware selector.

Update:
I have retried using the Firmware Selector for stable releases and it seems to work as intended now. I now also get a 'Unsupported target: x86/64' message for the RC3 build on the X86/64 target with custom packages. This leads me to think that they might still be building those as RC3 has not officially been released yet.