Keep in mind, defederation is bidirectional. You can end up on an instance that doesn't defederate anybody but is being defederated by some major instances and end up worse off. Also, communities are bound to an instance so even if your instance doesn't defederate with another, the instance that hosts the community might, which also doesn't solve anything.
Also lemmy.ml had to restore from backup monday because postgres shat itself, so if the post is from monday or around, it's possible it was simply lost due to the technical problems.
There's also some federation problems with 0.19.0 and 0.19.1, so it's possible it's been attempted to be delivered to lemmy.ml but failed due to load or whatever.
You didn't give any details or examples so we can only speculate. We troubleshoot federation by establishing patterns, like from what instance are the missing comments from, what instance hosts the community.
Addendum: I've also been experiencing occasional ghost posts, and I'm on my own instance, so there might be some stuff going on that's unrelated, because I sure didn't do anything. If they were deleted or retracted I would see them because I'm admin, I see everything.