Juniper MX : /31 issues

I've been using Juniper MX104's as PE's for a while and they've worked great. We use them to terminate fibre circuits into our core. Each fibre circuit comes in on a 10Gb which is split into 100's of VLAN's, one for each customer.

To save address space we tend to use /31's for these all has worked well fine for years now.

In the last week however I've had two instances where a customers circuit hasn't come up. I can see their device in the arp cache but I can't ping across it. Changing the link from a /31 to a /30 has resolved the issue both times or strangely moving the circuit into a VRF instance on the MX resolves the issue.

Now we have 100's of other customers on exactly the same 10Gb link and the CPE device are the same (managed by us).

I'm thinking it may be a bug on this specific MX104 we haven't hit before but before I start looking at Junos upgrades I just wanted to check in here. Anyone see this sort of behaviour before?

Clearing the arp-cache didn't resolve it either.

Thanks