Enabled Non-Linear adaptive coding enhancer (NoLACE) when (and only when) decoding opus in MP4 container.
libopus >= 1.5 built with --enable-osce (which is off by default) is required (binary attached here).
Unfortunately, ordinary .opus file is supported only via libsndfile, in which case qaac has no direct control over libopus. Hence only MP4 file is supported.