SQL Server Installer appears to be resetting MSDB Recovery Mode to Simple
This is already mentioned in the documentation at Microsoft site as below:


  By default, msdb uses the simple recovery model. If you use the backup
  and restore history tables, we recommend that you use the full
  recovery model for msdb. For more information, see Recovery Models
  (SQL Server). Notice that when SQL Server is installed or upgraded and
  whenever Setup.exe is used to rebuild the system databases, the
  recovery model of msdb is automatically set to simple.


You need to change the recovery model after doing any installation or service pack as per your requirement.

Hope above clarifies your concern.