[MC-148020] The observer back texture changes too many pixels when activated

Some grey pixels on the left side of the observer_back change when the observer is turned on, when they probably shouldn't.

Linked issues

Affects 1.15.2 and 1.16 pre2

MC-173310 has been marked as fixed in 1.16 Pre-release 3, but actually duplicates this issue.

Cannot Confirm/Reproduce in 20w28a, I think this is fixed.

The pixels marked on the screenshots don't change at all when the observer is turned on.

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