Bitcoin Core Wallet doesn't accept my passphrase
Are there any circumstances under which the wallet would behave like this?

If your passphrase contains non-ASCII characters, then you could have an encoding issue. I would try typing in the pass-phrase rather than cutting it out of Notepad and pasting it.
Unicode has multiple normalisation forms and this is another thing that could theoretically trip up software.

Could my wallet be corrupt ...

Yes, but your backups show the same result. If you successfully used the passphrase anytime after making a backup it is very unlikely that wallet corruption is the cause.

... and if so can it be repaired anyway?

No-one can tell you that. Repairing an encrypted file is improbable.