It behaves exactly like I would expect it to. It returns the appropriate format given the accept type of the request. Why should this cause problems?
It can only make things better as 3rd party client apps that cannot deal with it, might be encouraged to set proper accept types.
As far as the #Fediverse 's usage of #WebFinger is concerned —
Fediverse software (including Mastodon, Pixelfed, Calckey, Pleroma, Lemmy, etc) don't seem to use the #JRD “property” element.
But if any of them ever do — since the "type" is supposed to be a URI — I think it would be better to use something like a #tagURI or something like it (rather than an HTTP(S)-URI).
Else it may suffer from #linkRot (just like ostatus·org has).
Using Web Host Metadata's #XRD & #JRD to figure out where #WebFinger is seems unnecessarily complex to me.
(This is used by some #Fediverse software.)
...
/.well-known/host-meta
/.well-known/host-meta.json
...
