This might have something to do with the fact that Brave blocks third party cookies by default. The OAuth token used in TokenBB seems to come from buildteam.io, instead of the expected test.tokenbb.io.
I am using chrome 72 with nano adblocker and third party cookies disabled, which resulted in similar behavior (I used the magic link instead of Keychain). Allowing third party cookies resolved the issue for me.
Yeah the token comes from buildteam because we use a single sign on system where you are logged in in buildteam and all our apps have you logged in automatically. There sadly is no way around it but I will check if we can somehow whitelist our cookie as same origin
I would like to invite you to our Tokenbb-test-group channel in Discord, looking forward to seeing you there.