Hi,
I have just upgraded to Macbook Pro M1 and Traktor Pro 3.6.1. I have experienced this problem before in May 2020. Suddenly I couldn´t load a new track into deck A while playing. I know a workaround is to untick the box "Only load track into stopped deck" but it is a bit risky when playing Live. In Traktor Pro 3.4.256 I haven´t experienced the problem recently. I have ticked "only load song in to stopped deck" and "stop playing track at the end" I use Traktor 3.4.256 as stable version.
I thought this problem was solved a long time ago, but its not.
Best regards
Mikael