Bugzilla – Bug 1884
ICMPv6 does not check if the source address is "compatible" when issuing a reply
Last modified: 2014-04-12 16:23:53 UTC
In other terms: you can ping a global address from a link-local address and you'll receive an Echo Reply. I'm not 100% sure that this is legal. I'm fairly sure it isn't, indeed.
RFC 4443 doesn't say anything about it. As a consequence, I have to assume that this behaviour is not forbidden.