[MC-216089] Skulls are sometimes not visible trough stained glass

I have noticed that sometimes the creeper and skeleton skulls are not visible anymore while looking at it trough stained glass.

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Invalid, your game is modified

Thank you for your report!
However, this issue is Invalid.

Your game, launcher or server is modified.
If you can reproduce the issue in a vanilla environment, please recreate the issue.

Any non-standard client/server/launcher build needs to be taken up with the appropriate team, not Mojang.
Any plugin issues need to be addressed to the creator of the plugin or resource pack.
If you have problems on large servers, such as The Hive and Hypixel, please contact them first as they run modified server software.

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How is this modified and how do you know it is?

Optifine is a mod. See the environment