If a bootable AV can access the decrypted contents of the encrypted hard drive (e.g. by knowing the password for decryption) then it would be a security flaw. If an AV can do it, so can anyone else.
Bootable AV (e.g. via recovery disk) will not be able to access the decrypted contents of the hard disk which is protected - there won't be any decrypted contents for it to try and access because you'd need to firstly know it was encrypted before you attempt to decrypt and would need to calculate a key to use for the decryption using a password.
However, if you're referring to simply a normal AV solution starting up at boot, then it will work fine because that is after the initial decryption procedure at boot (since only after the decryption procedure will Windows truly start up properly -> eventually you reach the AV kernel-mode software starting up early -> user-mode services for Session 0 -> login and the auto-spawned GUI in the background if it does that).
So unless you meant like those bootable scanners, then everything should work fine in theory.