Hi,
thank you for reporting back on this.
To be honest, it is rather unsatisfying not to know what happened here. Based on my
experience, I am tempted to blame Suricata for this DNS issue, since it seems to drop
UDP DNS traffic sometimes (without logging it!), but I have no evidence for that.
Unfortunately, we have had to downgrade Suricata to 5.x again in Core Update 153 (see
this commit and this thread for details), so in case
Suricata 6.x fixed the error you experienced, I am afraid you will observe it again.
As soon as there is a (hot) fix available from Suricata, we will ship it’s latest version,
since it comes with some important security fixes as well. But 25 % idle CPU load is
just too much, especially on smaller systems. 
Apart from that, I am glad you like IPFire. 
The wiki isn’t very detailed.
What information were you missing exactly? Or is the navigation through it tricky?
Thanks, and best regards,
Peter Müller