I can't relate; doing this from OpenBSD6.7 to OpenBSD6.7 the ecdsa forward through and show up via ssh-add without any issues (and allow using the intermediary host without having the keys present (and being able to choose keys as per the initial question).
I was also able to do this over to a MacOS system which also handled the scenario excellently as described above and handled ECDSA just as well.
