On QFX5120VC or QFX5200VC, the license file of backup FPC would be disappeared after the upgrade. This behavior is seen when you upgrade the software version across 20.3R1 since Juniper Agile Licensing has been supported.
This KB describes the details and provide a workaround.

On QFX5120VC or QFX5200VC, there is a case that the license file of backup FPC would be disappeared after the upgrade. The issue could be seen after upgrade the software version across 20.3R1 and later releases.

This is coming from the Agile Licensing support implemented in 20.3R1 below.
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Software Licensing
Juniper Agile Licensing (QFX5120 and QFX5200) —Starting in Junos OS Release 20.3R1, we’re moving toward license-based software features. We now use Juniper Agile Licensing to support soft enforcement for software features on the listed devices.
Juniper Agile Licensing provides simplified and centralized license administration and deployment. You can install and manage licenses for hardware and software features using Juniper Agile Licensing.
From this release onwards, you can now opt to use the Juniper Agile License Manager to significantly improve the ease of license management for an entire network of supported devices.
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This change could bring licensing sync issue between master and backup FPC. So new master couldn't get proper licensing information from backup and could lead license lost. This is a kind of limitations and we'd recommend you to reinstall license when you noticed license lost.
NOTE : Even if you lost the license, there would not be a service impact on QFX as it is a soft enforcement license.

And if you save the license on QFX in advance, it would be easy to reinstall license.
Here is the procedure:
Step 1) Put license file to QFX.

-rw-r--r--  1 root     20     686 Aug 28 17:24 license-fpc0.txt
-rw-r--r--  1 root     20     686 Aug 28 17:25 license-fpc1.txt

Step 2) Install license when you lost it

root@Switch> request system license add <filename>
Possible completions:
  <filename>           Filename (URL, local, remote, or floppy)
  license-fpc0.txt     Size: 686, Last changed: Aug 28 17:24:53
  license-fpc1.txt     Size: 686, Last changed: Aug 28 17:25:08