For those who are playing at home, two questions across a few examples should illustrate the frustration:
Q1. What should happen?
Q2. What do you think would happen with implementations today?
Scenarios.
S1. Replying to two separate posts.
S2. Replying to itself.
S3. Providing a Link object reference.
S4. Providing a Link*
S5. Providing an Actor.
S6. Providing an Activity.
This might simultaneously amuse and/or horrify you, from an SRE perspective, but a "chaos monkey" type ActivityPub implementor that randomly shuffles through all possible permutations of data structures for any given object... would be good for the ecosystem.