You should always have access to the DLC you purchased because you paid for it.
A workaround was posted on r/PS4 by Reddit member SegaJAM, and it has helped many people in the comments save their DLC and sanity.
After the firmware upgrade, recovering DLC is a straightforward four-step process.
Hold the PS button on your controller while looking at the PS4 dashboard.
Select Stop Using Extended Storage after going to Sounds/Devices.
An notice will appear allowing you to unplug your external hard drive, but you are not required to do so, so don't bother.
Choose Power > Restart PS4 while holding down the PS button.
Await the PS4 to do a new hard disc scan.