In this case, it doesn't sound like they're reverting it because of overall breakage, but rather because it breaks the tool that would otherwise be used to control TLS 1.3 trials and other configuration. Firefox had a similar issue, where they temporarily used more conservative settings for their updater than for the browser itself, to ensure that people could always obtain updates that might improve the situation.
It sounds like a perfectly reasonable behaviour if the goal is to "fail closed", to provide more security in a fashion similar to a whitelist.
If it sees that it's TLS, it should attempt a protocol downgrade.
I don't remember the exact details but I recall reading that TLS has a mechanism to prevent version downgrades, precisely to defend against such "attacks", so the connection would not succeed in that case either.
If merely advertising 1.3 while still advertising older versions causes blue coat to break, it has a bug in TLS version negotiation.
There is no downgrade or whitelist or failing closed. Each end says what they support and BlueCoat blows up the connection if it sees that the other end supports a newer version. It should say "oh we both support 1.2 let's use that" And apparently it's done this before so there's even less an excuse for it.