NeoForge version should be able to accept "-beta" suffix
Users attempting to provide a NEOFORGE_VERSION with a beta version like shown on NeoForge's website
however the forge installer command rejects the version for being invalid.
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So you don't seem to need the -beta suffix to make this work. With some trial and error today I managed to get neoforge pinned to 21.6.12-beta by using NEOFORGE_VERSION: 21.6.12 instead of NEOFORGE_VERSION: 21.6.12-beta
So I wouldn't call this a bug, but an opportunity for improvement, since users naturally will try to use the version label with -beta appended, since that's how it shows up on Neoforged.
Thanks for the research @Adavee
I'll re-open and tag as an enhancement.