If the spectator goes inside a slime while being in third-person mode, they can't see their own head even though the slime's outer layer is translucent.
This is probably a duplicate but I don't know what the cause is so it makes it difficult to use the right search terms.
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Thank you for your report!
We're tracking this issue in MC-161885, so this ticket is being resolved and linked as a duplicate.
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Duplicate ofย MC-161885.