This issue may be exclusive to the DDJ-SZ2, or maybe it impacts other hardware too.
Problem: In Sampler "Master" output mode, Serato splits the Sampler volume output across all 4 channels. The hardware mixer then sums all 4 channels. So the Sampler volume is multiplied by all the fader positions (good thing they're all in phase, or it'd cause another issue).
Suggestion: Output the Sampler output to its own channel or to a true master output (post-fader).
Meanwhile, we are having to use specific channels for the sampler. This is a distraction & is limiting in how we use other channels. This is not a fix, not even a band-aid really.