I had the same problem with a fresh install of :
Mac OS High Sierra
French language and French numeric keyboard layout
APFS (case-sensitive & encrypted)
Enabled FileVault
Installed BootCamp
After the BootCamp installation, I restard to set the default boot to Mac OS but I stopped on the FileVault password request for my account.
It wont recognize my password... On the top-right corner, it show "French - Numeric" keyboard layout is active but in fact it is not... I have to chose the "French - PC" layout and type like it was a PC keyboard to make my password works.
So, for me, it is not a problem of cryptography but simply a problem of keyboard layout...
Try some other keyboard layouts and test your password again and again.
It is way harder to find a working keyboard if your password contains symbols.
Good luck