Originally Posted by
Rowyourboat
I ran into that too, very weird. What I did was search for ADD->DUB, select Ethiopian, then go back to searching BKK->YYZ, and was able to keep the Ethiopian filter.
Google Flights is quite buggy with advanced functionality like that (you can usually edit those filters in the URL), as it's really just a user-friendly skin for ITA Matrix.
I found these fares no problem on ITA, then replicated on both an OTA and the ET site.
In my experience, Google Flights is great for exploratory stuff, but to drill down into the unusual you really have to use ITA with advanced routing.