There is a report on the mailing list. We should clearly define the rules and we probably need to fix gpg.
gpg tries to find the "best" key using get_best_pubkey_byname (https://dev.gnupg.org/source/gnupg/browse/master/g10/getkey.c$1507), but the applied rules are not clearly documented in one place.
I don't think that we need to fix things here. Important is that the WKD import uses a filter which imports only keys with the requested mail address. However, if a key with the same fingerprint already exists it will be merged.