Uhm thanks! Then it’s weird. But did you try to display the output to an external monitor? Because also for me was working if launched from my computer, not on the script that runs locally on the RPi.
Or maybe it’s because I used the code as a variable.
boot_time=$(systemd-analyze | awk '{print $NF;exit}')
Or maybe I simply done an error when I inserted inside my script 
I’ll investigate it later at home.