No links permission not being obeyed. Why?

[CrispinP]

I've getting a lot of spam at the moment with users who registered months ago and have never posted.

• They're in the Registered Group which has Allow Links set to No yet they can still post links.
• They're not in any other group.
• If I test permissions on the user then it shows as Allow Links as No.

Edit:
Signature also has no links allowed but they're doing it there too.

Why would this be happening?
No links permission not being obeyed. Why?

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