impossible to expand the stack, or to unstack

"When capturing the frame in a video, it appears as if it were stacked, but it is impossible to expand the stack, or to unstack."

You may have a filter that's hiding the captured frame. Make sure the menu item File > Library Filters > Enable Filters is unchecked.

It that doesn't help, please post a full-resolution screenshot of your entire LR window showing the problem.

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