@mduchaine 

Just thinking out loud here, the MDNS packets are going to be forwarded via the AP to whatever it is connected too, you mentioned this is your gateway?

Could your gateway have a config or firewall setting to drop these packets?   Being honest its unlikely but if you have an un-managed switch somewhere remove the gateway leaving only the AP / Controller and Switch, see if it works then..

Also you mention you are running v3 of the controller software, the EAP245 is better supported running v4 (4.2.8 is latest) of the SDN software.  People have reported a number of strange issues and these have been related to the controller on v3, usually things like stats not reporting or traffic monitoring not working.   Upgrading this might be an option also

@Philbert 

My gateway is actually a firewall running Avahi and forwarding packets onwards to other networks.

However in this particular case both laptop (querier) and devices exposing mDNS services are on the same wireless LAN; same AP, same SSID, same network subnet.

I do not expect mDNS traffic to go through the gateway at any point.

I mentioned the gateway only because it allows me to see the devices are actually responding to my laptops' query.

You were probably right at pointing at some form of "bug". I have a scheduled reboot of all APs once a week and yesterday replies started to show up.
Looking at upgrading my EAP245, my omada controller reports the v3 to be at the latest version.

Why is my controller not seeing the latest version? You mentioned 4.2 - this one is still at 2.4

@mduchaine 

Im guessing you are running the Software controller, however the same is true for the OC200/300

If you are using the v3 of the SDN and you choose to check for updates, it wont offer you v4 (only the latest v3).  This is due to the v4 update being a large change overall and some device support dropped, therefore you cant auto update to it.

To install v4 download the update from the TP Link site, look for SDN 4.2.8 update and point the controller to this file..

Info and screenies in the lin.

https://community.tp-link.com/en/business/forum/topic/246396

Just note to do a backup of config/settings before updating and i would recommend un-adopting your EAP245 then re-adopting after the update.

Best of luck