[j-nsp] icmp-tunneling not working?

[Cydon Satyr]

So I have a simple bgp-free core network, and one CE router is trying to do
traceroute to another one over mpls. Two of my middle P routers don't reply
back to traceroute.
I have icmp tunneling configured in my whole network. So it's global IP,
one mpls label.

In fact I can see running a wireshark that both P routers receive a mpls
packet with ttl=1 but looks like they don't do normal "icmp-tunneling"
procedure and simply drop the packet.

What's more is that in another part of the network (path going over
different set of P and PE routers) this works just fine, yet I fail to see
any difference in config which could explain the failing behavior.

This is a testing topology with a mix of M320s and MX, but I could see why
a customer would like an explanation of why he doesn't see proper
traceroute.

Before I reload both of my P routers, could anyone suggest what could be
the issue?

Regards