Yep that’s indeed what “fixes” the issue. For FP2 we’re missing some license for getting Qualcomm’s acoustic echo cancellation actually activated. So if you load their effect libraries, AEC breaks, but if you don’t load them (as it happens in stock FP2 A9), Android falls back to a software implementation that works well enough.
In Android 7 we had some patches ready actually to disable AEC somewhat cleanly, but they didn’t make it into a release anymore. See here for reference: https://gerrit-public.fairphone.software/q/hashtag:%22aec%22