Workaround
When you rename Opus file to M4A, it is imported, but the wrong waveform is shown and silence is added in the end of the file (sometimes it stops happening), although the file is exported correctly.
To fix the issue with additional silence, you can use MKVToolnix to place Opus into Matroska container in order to create MKA file (no reencoding is performed, bit stream is copied).
MKA is not supported either, but now if you rename it to M4A, the correct waveform is shown.
I reported it to customer service.
