How to match ICMP Destination unreachable (Port unreachable) to original message

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[VirtualBox 5.1.30 r118389 (Qt5.6.3), HOST: MacBook Pro High Sierra 10.13.1, GUEST: Ubuntu 14.04 LTS]
[Wireshark 2.2.7 (v2.2.7-0-g1861a96)]

I was having some local domain resolution problems with a DNS server I have setup inside a VirtualBox 'Host-Only' network that I think I've now fixed. The server would sporadically resolve my local (internal access only) domains but later refuse to resolve them (only to recommence resolution some time later). I used Wireshark to help me debug the above problem. The symptom of the problem seemed to be that my HOST was generating a lot of ICMP Destination unreachable (Port unreachable) packets that were being sent back to my DNS server.

Over the last few hours, my DNS resolution seems stable and the number of these packets has reduced significantly. Whilst things are looking much better here, I am still getting a few of these packets periodically and I would like to fix the issue that is causing them, whilst I have the momentum.

Whilst investigating the problem and searching the web, I came across a short thread in google groups: 10.6 DNS resolution, does not obey DNS server priority from DHCP that intrigued me. Of particular relevance is a thread which Andy references, and I'm wondering if this issue with mDNSResponder, or something similar, still exists as I believe it could explain the problem I'm having.

10.6 (Snow Leopard) all DNS queries now go via mDNSResponder. mDNSResponder queries for both the "A" and "AAAA" record, but after the first response it cancels the queries and shuts down the socket so that all other responses are rejected (port unreachable).

In my case, I have IPv6 resolution turned off in my DNS server, so I'm only asking for A records. But I'm wondering if mDNSResponder is closing the ephemeral ports before the responses are returned, a kind of accounting problem? Not being a regular user of Wireshark, I am having difficulty matching the requests with responses, so my question is:

How can I match the ICMP Destination unreachable (Port unreachable) error report packet from the HOST to the packet that cause the error; ie, which query response was not delivered? (See black/selected blue packets in the image below.)

(URL to the image is: https://abms.net.au/public/PortUnreac...)

[UPDATES]
Here is a breakdown of such a packet, and from what I can see, it is not following the RFC792 format, as per the example shown below. Contrary to RFC792, the 1st 6 bytes of the packet are the Mac Address of the Server hosting the DNS....

Here is the data in a ICMP Destination unreachable (Port unreachable) packet, as an image (couldn't figure out how to copy/paste it):
(URL to the image is: https://abms.net.au/public/PortUnreac...)

Here is Wireshark's decoding of the packet:

Frame 876: 70 bytes on wire (560 bits), 70 bytes captured (560 bits) on interface ...