the life bar disappears

Do you have any resource packs enabled? If so, does disabling them fix the issue?

I don't use packs, just the original Minecraft one

Do you have a copy of the world, where the issue can be observed? Can you attach it to your report? If it exceeds the file size limit, you can upload it elsewhere (e.g. OneDrive) and link it here.

https://www.mediafire.com/file/84azmsctt4uncgp/realms+world.mcworld.zip/file

I just downloaded your world from the link provided, but I cannot reproduce the issue. All bars appear correctly for me. Does this issue happen every time whenever you log in to your world or it's happening at random times?