MacBook Pro 2018 keyboard lights turn off after login

I would try resetting NVRAM/PRAM Reset NVRAM on your Mac - Apple Support

and compare your results.

no resolve Try resetting the System Management Controller Reset the SMC of your Mac - Apple Support

To trouble shoot further you can:

—A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

Login and test. Reboot as normal and test. Caches get rebuilt automatically.

This test will tell you if third party interference; most extensions etc are not loaded in safe boot mode.

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support

This will tell you if it a universal issue or isolated to your user/admin account. 

Check your System Settings > Keyboard panel for the 'Keyboard brightness' and the 'Turn keyboard backlight off after inactivity' settings.