Loose sprites:
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When these three selected sprites are grouped, it stops rendering correctly:
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Create or find two sprite assets
Place down two copies of sprite A and one copy of sprite B
Select the sprites in the order A B A
Click on the group button in the details panel
Expected: Nothing changes visually
Actual: All sprites after the first material or sprite type swap will be garbage (flickering on top of a previous sprite or missing entirely).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-84840 in the post.