If you used a running BookwormPup64 10.0.7 and used Frugalpup Installer that is in it.
You did use V41 of Frugalpup installer.
Something you could try just to see if it affects this.
Run the Frugalpup installer ->Boot
when asked what type boot loader to install.
Choose both.
It will install every type for UEFI bios and legacy bios booting.
Only thing you may see when selecting from the list of boot devices.
More than one listed for the same USB. (2 different boot loaders on the USB cause this)
Depending on the bios setup.
One listed will work and the other one will not.