I'm not sure you can classify this as a failure, as explicitly prohibiting interfacing with non-agpl stuff would greatly limit the amount of stuff you can license under it, perhaps up to the point of making it generally unusable. As for "not like that"... Well, yeah. But you can't deny it's misleading, right? Free software kinda implies you can modify it whatever you want, and if it's a free ui relying on a source-available middleware... Turns out, not so much.

Although, a posdible solution would be require explicitly mentioning if you're basically a front-end for something; but I'm not sure if it can be legally distinguished from the rest of use-cases.