This is a known issue that occurred at some point in MX17.X - not entirely sure which version, we jumped from 15.X to 17.X a few months ago and this started spamming our syslog. I opened a case with support, and to quote:
"We do not yet have a full fix for this in versions of MX firmware that have support for IPv6 (namely 17 and above) but version 16 does not have this issue."
The last update to that case was in February, haven't heard anything since.