MX Player Pro and Dolby Digital 5.1 audio

I have a Firestick 4K Max and an A/V receiver with surround speakers.

If I play a particular movie using the official Netflix app, I can properly hear the Dolby Digital 5.1 surround sound. So the HDMI and A/V settings seem to be setup correctly.

However, if I play that same movie using one of the other popular streaming apps, and configure the streaming app to use MX Player Pro as the external player, I do not get the Dolby Digital 5.1 audio even though the selected stream source says it features dolby digital 5.1. But, if I instead configure the streaming app to use it’s internal player (exoplayer), then I will get the dolby digital audio for that same streaming source. So, it definitely seems like it is a problem with the MX Player. But I tried every setting and still can’t get it to work.

Has anyone else been able to get MX Player Pro to work properly with Dolby Digital 5.1?