Based on MCPE-46047
The boss bars are still missing implementation for overlays/notches.
The BossEventPacket finally accepts colors at least, but as it is typical for Mojang, instead of also fixing / implementing the overlays, only the color field has been implemented.
[media][media]Linked issues
is duplicated by 1
Comments 2
Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
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Issue MCPE-143064 should contain all ignored values, including the overlay.