This happens if you have a newer version of Linux, trying to boot on Legacy menu instead of UEFI and your BIOS doesn't recognize partitions without the boot flag.
The solution is as simple as adding the boot flag on the ext4 partition, here's how I did it for Manjaro.
Run this on Terminal:
parted -l
Will list all the partitions.
Check if the ext4 of the USB you want to boot has the boot flag. Some BIOS need it to be able to run.
Model: SanDisk Ultra Fit (scsi)
Disk /dev/sdd: 62.1GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 1049kB 53.2GB 53.2GB primary ext4
2 53.2GB 62.1GB 8907MB primary linux-swap(v1)
So in this case, we want to update /dev/ssd and the partition 1 to enable flag boot.
We do this then:
sudo parted /dev/sdd set 1 boot on
This will fix the partition and it will work.