This bug is 100% reproducible in my device, not sure in other device.
My device is Nexus 6P, system is stock Android 7.1.1 (build number: N4F26T)
This bug did not ever happen in Poweramp v2. the very same MP4 file is played without any issues in V2.
Reproduce the bug:
download THIS MP4 file from youtube: https://youtu.be/uu3bD2Rk8hg (or use this url https://r1---sn-a5meknlz.googlevideo.com/videoplayback?sparams=dur,ei,expire,id,initcwndbps,ip,ipbits,itag,lmt,mime,mm,mn,ms,mv,pl,ratebypass,requiressl,source,upn&upn=Zxf2QQ6R2_c&mime=video/mp4&ratebypass=yes&source=youtube&pl=21&signature=26F756D8CF6C1285BAF3EE70B7F9A8CB8E9065CB.46DBEA95695D5F2C0BAB578E92DF60E133CCE3F6&expire=1491400902&ei=ZaTkWL3dOsiy_AOUyJOwDQ&ipbits=0&requiressl=yes&id=o-AFS9xXO-SDza1SdAl9IZvAG8XVjO5O5QIDYzaVT7Qbd6&itag=22&dur=127.941&lmt=1473019942736247&key=cms1&ip=45.79.74.181&title=%E3%82%A2%E3%83%9E%E3%83%84%E3%83%84%E3%83%9F%20OP%20-%20%E3%81%93%E3%81%93%E3%82%8D%E3%81%AB%E9%9F%BF%E3%81%8F%E6%81%8B%E3%81%BB%E3%81%9F%E3%82%8B%20@60p&redirect_counter=1&cm2rm=sn-n4vez7e&req_id=26834ccb764da3ee&cms_redirect=yes&mm=34&mn=sn-a5meknlz&ms=ltu&mt=1491379279&mv=m, but I'm not sure it can always be downloadable)
play this mp4 with Poweramp V3, when it plays to 1m18s, the play speed significantly drops, make the female voice sound like a male. Then even you backward before 1m18s, speed will still be unchanged. You can fast forward to or beyond this position. It will also trigger this bug.
This is not the only file which can trigger this bug.