First DNS request fails, subsequent work

[beneix]

I have been running migumail's AdGuardHome plugin for a couple of years on OPNSense, without any issues. Then around the time of updates from 23.7.10_1 to the next version, I suddenly started having a strange issue. For most attempts to access a site, the first attempt fails (screenshot attached). When you refresh the web page, or when a device (such as one of my Squeezebox players) attempts a second time to access a stream, then it works.

Subsequent accesses in the same session (or on the same day, I haven't quite figured out the pattern) all work as expected.

The issue does not happen for all web sites; I have not yet figured out what determines when it works and when it does not.

The thing is, when I get one of these failures, if I then check the AdGuardHome query log, it shows neither a blocked nor a processed query request, so it is as if that first (and failed) request never reached AdGuardHome. The subsequent (successful) requests do show in the AdGUardHome query log.

This is why I am posting a question here rather than in the AdGuardHome forums – I do not think the problem lies with AdGuardHome, instead I suspect there must be some setting in OPNSense that is wrong (and probably changed with one of the upgrades). What can I do to investigate the issue?