Relates to MC-165036, a boss bar texture that uses transparency doesn't show its transparency if other boss bars are present. The example in these images shows that the transparent black bar behind the boss bars doesn't render on all boss bars below the first.
Steps to Reproduce:
1) Use a resource pack that has boss bars with transparency
2) Summon an Ender Dragon and summon a Wither (the second one summoned will always have their transparency unsupported)
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16 more links
Please attach a screenshot of this issue occurring with the F3 screen enabled.