If you have a boss bar (tested wither and ender dragon) and you teleport, the boss bar disappears. This happens on latest version on 1.15.2 and with recommended forge version. No configs changed. Reloading the world fixes this issue. Also happens on latest version on 1.12.2 (tested to teleport to and from end via stones).

Seems to happen when teleporting while staying in range of the boss, as the boss will then not untrack you, but all boss infos will still be reset due to the teleport. This looks like it'd be quite difficult to fix, so might be best to leave it to Vanilla to fix on their end.