Not good, but can happen.
According to the install instructions the OS was flashed to both slots.
Just try fastboot --set-active=a and fastboot reboot.
If the phone doesn’t boot successfully, try fastboot --set-active=b and fastboot reboot.
If the phone doesn’t boot successfully there either, I seem to remember LineageOS had a script to really synchronise the contents of slot A and B with all the partitions not flashed when installing /e/OS in case something in that department was odd. To be found here (as referenced in LineageOS install guides).