"Failed to load/authenticate boot image: No Media" after running postmarketOS on SD card

So I've encountered this error after I installed postmarketOS to the SD card.

As I'm testing the OS on the Fairphone 5, I noticed the FP5 instructions on how to do so were out of date.

First I installed it to the SD card with pmbootstrap install --sdcard=/dev/sda, once it was installed on there, I removed the SD card and placed it into the Fairphone 5. I then ran pmbootstrap flasher boot as according to the up-to-date instructions.

It worked, and I booted into postmarketOS. Awesome! So I did some testing, then I decided to reboot and switch back to my Android OS.

And then that's where the troubles began: It wouldn't boot past the fastboot screen. So my next option was to try and boot a TWRP recovery image.

Did it work? Nope! This is the exact error I got:

administrator@MS-7984:~/Downloads/platform-tools-latest-linux/platform-tools$ ./fastboot boot ../../twrp-3.7.1_12-0-FP5.img 
Sending 'boot.img' (98304 KB)                      OKAY [  2.050s]
Booting                                            FAILED (remote: 'Failed to load/authenticate boot image: No Media')
fastboot: error: Command failed

Now I'm really curious as to how to fix this issue. I've searched everywhere online for this, but this error was completely non-extistent and unheard of.

And just to be absolutely sure. Yes, my Android partitions are still there.

I've tried fastboot getvar current-slot to get the current slot (which was slot b), then I did:

fastboot set_active a
fastboot set_active b

And it didn't work at all. I've even tried taking out the battery and putting it back in. Nothing worked.

The only thing that I can do is run pmbootstrap flasher boot, but that only boots into postmarketOS.

I've even tried flashing my backed up boot partition to the boot partition via fastboot, and it still doesn't work.

So I'm really curious if anyone here has had this issue and if there's any workaround to this issue?