I don't have the explanation, but at least I can confirm that this behavior is not specific to /c/Belgium, and it can be the other way around. I was browsing a community hosted on a remote instance. When browsing from lemmy.world, I could see several posts, from lemmy.world users, which were not visible on the community when browsed from the remote instance hosting that community. And those posts hadn't been removed on the remote instance after publication ; they only got published on lemmy.world's copy but not on the instance hosting that community.

You're right, but I only figured that out after the first fetch from r/explainlikeimfive from your bot. Feel free to remove /c/explainlikeimfive from your communities, to avoid wasting resources, and to prevent Lemmy users from replying in it instead of posting on an actual new c/explainlikeimfive on another Lemmy instance. I think I'll submit subreddits which have much more value in OP's initial submission, such as r/interestingasfuck or r/todayilearned. Thanks for your work!