YSK there is a setting in your Lemmy profile to hide posts that you've seen. Upvoting counts as interacting with that post and will hide it after refresh.
Also, if this selected, it will hide any posts you make. I have been saving the posts I make as a favorite until Lemmy fixes this.
Yes it does. It's a hard setting for your Lemmy profile that is applied to your account, which effects your apps. Some apps allow you to change this in the settings.