I have mark as read on scroll activated.

Home is set to top 24 h.

I reload my home feed. The first post is not marked as read.

Go to "You" tab. Open one of my own posts.

Return to home feed. The first post is marked as read although I didn't scroll past it yet.

Edit: Just saw that I can also mark the first post as read (sometimes also the second) when I simply rotate my phone to trigger a screen rotation. It does not always happen on the first rotation.