I've had several chests and signs go transparent. Chests are emptied and unusable. If you break the chest, you get it back, but contents are gone. You can see the "wire" outline of the chest.
Cannot see previously created signs but you can see the wire shape of it. If you break it, you get a sign back.
Other signs and chests remain intact so far. Cannot determine a specific locale or action/cause of these disappearances.
Example of "wire" sign in attachment.
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The cause: block entities are gone, the trigger I cannot tell