Not sure the origin of this issue; it looks like certain files that are on the list of files in the release that Twitch provides to the launcher are not found. Depending on the version of Forge above 0.16 I pick, I will get different files not found. This happens after the profile is created and you're in Mojang's launcher, but before minecraft itself loads. Additionally, sometimes when launching these versions of Forge, it fails saying it could not verify the version of minecraft.
Also beware that you don't get the 'latest' version for mods that have both Fabric and Forge; Twitch doesn't have any idea that fabric is incompatible and if the fabric version is newer, will get it instead.
Anyone know where to lodge official complaints? Not that they seem to be paying any attention.
confirming that files are not being downloaded for forge versions greater than 0.16. I can't even make a new instance for 0.17. For official complaints it might be a forge issue and not a twitch issue.