Micro.blog users in Micro.blog responding don’t generate an ActivityPub response because that’s not what they are.
Just go over how this works/the options:
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Micro.blog with an ActivityPub “feed” – replies can come via micro.blog to the post in micro.blog nothing to do with ActivityPub or people following the ActivityPub feed from Mastodon can reply, they’re replying to an ActivityPub post, it will show up in Micro.blog from their Mastodon account. All of the replies to a Micro.blog post this way is seen within Micro.blog and can be replied from there.
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Cross posting to a Mastodon account, replies to the Mastodon account will be in Mastodon as ActivityPub replies over there. But the original post in Micro.blog is disconnected and in Micro.blog as it’s own thing. Replies to that post from Micro.blog won’t show up in Mastodon because it has nothing to do with the cross posting account.
The core “problem” is if you post on Micro.blog from a Micro.blog account, there’s a representation of your blog post on your Micro.blog timeline that is totally separate from any notion of ActivityPub. If someone follows you on Micro.blog, it’s not an ActivityPub / mastodon situation. Micro.blog itself is fundamentally not a Mastodon instance.
You can unify all replies to a Micro.blog post to being viewable in Micro.blog using an ActivityPub compatible Micro.blog user name. But you can’t unify to only have things exist on Mastodon without turning off having any Micro.blog feed.