Watch your PIDs for changes to the niceness after they've started.

watch-nice-changes.py

import time
import psutil

nices = {}

while True:
    time.sleep(0.2)
    for proc in psutil.process_iter(['pid', 'cmdline', 'nice']):
        try:
            cur_nice = proc.nice()
            if proc.pid not in nices:
                nices[proc.pid] = cur_nice
            else:
                if nices[proc.pid] != cur_nice:
                    print(f"Process changed priority at {time.time():.2f}: {nices[proc.pid]} to {proc.nice()}, {proc.pid}: {proc.cmdline()}")
                    nices[proc.pid] = cur_nice
        except psutil.NoSuchProcess:
            del nices[proc.pid]