I can confirm this using Piefed rather than Lemmy. I don't have the issue on Lemmy.
post
I've seen this occasionally too, but always assumed it because my instance was de-federated from the commenters instance.
replies:
Does pulling down to refresh not update it for you?
replies:
permalink
fedilink
source
parent
hideshow 2 child comments

Here's an example. 28 comments total, but only 11 are visible to me in Boost.

all 9 comments