When glass is placed in an item frame, you are able to see through the whole thing, and see whats behind it.
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Graphics card/driver dependent issue as most of the recently reported rendering issues are.

Please force a crash by pressing F3 + C
for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) here.

In what graphics mode does that happen for you? Fast, Fancy, or Fabulous!?
Cannot reproduce in 1.16-pre3 ?