Ender dragon, its health bar and exit portal didn't appear when I enter the end

I went into the End by placing an end portal block (command: /fill -56 15 1805 -56 15 1805 minecraft:end_portal). When I went into the End, the ender dragon health bar, ender dragon, exist portals had disappeared, photo attached in the issue. I had searched for about five minute and the health bar didn't show up.

What I expect to happen was...:
When I went into the End, I will find out the health bar showing, ender dragon flying, exit portal in the middle

What actually happened was...:
None of the things that I described appeared, except ender crystals and endermans

Steps to Reproduce:
1. place an End portal block by the command: /fill x y z x y z minecraft:end_portal
2. get into the portal block
3. notice there are no exit portal, ender dragon, and ender dragon health bar