Hi Cillus,
I have tried several different permutations of using %3A, ‘:’, urlEncode(‘:’), and other methods and I can confirm what you are seeing. I get a 404 in postman and a ‘no Oauth Credential’ in postman, which is what I would expect. I do know that some behavior in encoding/decoding URLs changed in the release of 8 as outlined in this release note:
We fixed an issue in consumed REST services where encoded URLs were decoded too early. We upgraded httpclient.jar from 4.5.7 to 4.5.8. (Ticket 83346)
So I would indeed recommend that you create a ticket with support for this.