License keys are tied to the hardware GUID of the WFE server and if the GUID changes, the license will revert to the trial status. You will need to deactivate the license key and then re-activate the license using the same key.
GUIDs may change when a WFE server is dropped from a farm and then re-added.
IMPORTANT: For products licensed with the Bamboo License Manager (rather than the Web License Manager), the license key is tied to the hardware profile of the PC where the product is installed. If the hardware profile changes (i.e., a hard drive is added), the license will revert to the trial status. You will need to deactivate it and then re-activate it using the original license key. Products that use the Bamboo License Manager include: List Bulk Import, List Bulk Export, and User Profile Sync.
The problem is that the license status will no longer show the license key once the GUID changes, it will show as an expired trial license. See Is there a way to find a lost or unknown Bamboo product license key and then How to move your Bamboo Product License to a new server to resolve this issue.