RFC 2119:
"
SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
In other words:
"Should NOT" != "Must NOT"
"Should NOT" == "Not Recommended"
So, the upstream router is not doing anything wrong but it is doing something "not recommended". It appears that Juniper have elected to make the "should not" a "must not" which of course they are entitled to do. By the fact that everyone else in the same building does not have this problem, I suspect that I am the only one with a Juniper firewall (I know there's a lot of Cisco in the building). Also, as I say, I know that Windows firewalls do not have this problem.
I will of course speak to the upstream guys anyway, but my guess is that they will say that they're not doing anything wrong (just not recommended) and that I should talk to Juniper. Sigh. On the positive side of things, they're only small SRX's so it's not as iof there's been a major spend on this.