Different public keys of the same actor MUST be identified using different fragment IDs.
This statement could be confusing in scenarios where fragment key IDs are not being used. For example, an actor could have multiple public keys where none of them use fragment ids or they might use a mix of fragment and non-fragment ids. The statement implies all keys MUST have a fragment id which isn’t consistent with the SHOULD recommendation for using fragment key ids in the previous sentence.
I think it would also be useful to provide a rationale for strongly recommending that the key id is based on the actor URI with a fragment. I understand this is the common pattern in the Mastoverse, but there are valid scenarios where this is not desirable (external key storage, for example).