This post contains a small experiment: a counter which should increase every time this post is opened. Or maybe not, depending on caching. It uses count.ayaya.beauty; hosted by nadeko, a provider of privacy focused services.
Sauce: Zenless Zone Zero
Characters: Jane Doe
My guess is that most voters are not opening up the post and viewing the counter. Basically every lemmy frontend I can think of and every mobile app let you view the image of an image post as well as vote without opening the full post and seeing the post contents. Piefed works similarly as well and will show you the image as well as let you vote without needing to load the full post body.
As for fedia, I suspect that it is falling prey to the same issue that was plaguing piefed (until we just fixed it recently after talking on another one of your posts). Creating the post and editing it so soon afterwards probably works just fine on your home instance because there is enough temporal separation between those two actions that your home instance can deal with it. However, fediverse software tends to batch outgoing federation activities, so both the create and edit activities are getting sent to other instances in the same batch. Without some way of putting the edit activity after the create activity, the edit will fail before the create completes.
I agree that most voters just vote from their feed without opening the post (this is also how I view content here). That is why I am surprised that the counter is much higher that the votes, as I expected the opposite (+ counts from instance auto caches). Then again, there might also a number of crawlers which consume posts on the Fediverse.
Regarding fedia: I too suspect the same issue as with Piefed. I wrote a post in the support community of fedia.io and mentioned one of the Mbin devs.
Also it seems that I hopefully don't have to edit the post after its creation in the future anymore, as there is a PR for Mbin which will enable image-uploads + body via the API (currently the API is quite odd that a thread can be created with either a image or a body, but not both with the same request).