I suspect that this may be because the tab hasn't loaded / hasn't finished loading yet (since the tab preview is not shown for the closed tab). Thus, fenix doesn't consider it as "opened" to begin with. Can you check if the tab is saved in the history before closing and checking if it is in recently closed.
Bug 1918857 Comment 5 Edit History
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I suspect that this may be because the tab hasn't loaded / hasn't finished loading yet (since the tab preview is not shown for the closed tab). Thus, fenix doesn't consider it as "opened" to begin with. Can you check if the tab is saved in the history before closing and checking if it is in recently closed please?