I just tried the simplest sequence and it did NOT fail. It worked. I will keep an eye on it and try to figure out what sequence causes it to fail.
Here's the sequence that worked fine:
1. Open an instance of Media Monkey
2. Open a second instance of Media Monkey
3. From the second instance, Hit Play
4. From the second instance, Navigate to and select: Play > Stop After Current


Windows 10
MediaMonkey 4.1.20.1864

Hi,
Can you please elaborate, I'm not sure I follow it correctly as I can't replicate.

When two instances of MediaMonkey are open, Stop playback after current file does not work from the second MediaMonkey to have opened.