I can’t say for certain but the fediverse is prone to behave unpredictability at times. You’re probably better off thinking about Lemmy as a collection of websites rather than one central hub like reddit or more traditional sites.
Instances go up and down all the time as well. Sometimes when a site goes down for maintenance (and probably other things like sync issues) the subscriber numbers will fluctuate because a bunch of them are on a site that “doesn’t exist” for a short time while it’s being worked on.
Everything you do on Lemmy takes longer than what you’re probably used to on a more traditional website too. Your posts to your local instance should be instantaneous but sometimes it can take minutes, hours, or even days for those actions to fully federate to other places. This is true of posts, replies, votes, and subscriptions.
There is also defederation which can cause the behavior you described. You are on one of the largest instances on Lemmy and some people have soured on it and its users. It’s entirely possible some of your subscribers were on an instance that defedderated with lemmy.world or something.
TLDR: You are likely overthinking the impact of bots and putting too much importance on subscription numbers. Make sure the community you care about can actually reach the people who might be interested and hope for the best.
Only slightly related but maybe you’d be interested -
I put together a help post for new Lemmy users on my instance describing how to find and join new places on Lemmy. Maybe these resources will help you understand more about how Lemmy works and how you can get your community off the ground.
Maybe it’s part of your instance image policy?
Edit - Need coffee and weed before getting on Lemmy. I can’t read apparently and missed that you tried this on your own instance as well. My bad.