sleep/lock button not working after screen assembly replacement

I have one at my shop doing the same. I believe it may be related to the small piece on the bottom near the home button that requires soldering. It is the same as the iPad Air 2. Did you transfer that piece and solder it onto the new screen? This may be the answer

I changed the screen for the ipad mini 4 and didn't realize the bottom ribbon cable was soldered to the connection. I heated it with a heat gun and was able to remove the cable from the connector with a pair of tweezers, but I had to use extreme caution. it took a long time to get it loose. I actually broke the connector on the top getting it loose. Once loose, it used a magnifying glass and pair of tweezers to push the end of the ribbon cable into the connector. holding my breath, I connected the screen to the motherboard and tested it and luckily it worked and the button also functioned normally. I don't think I will be doing any of these replacements as it was extremely time consuming. Best of luck to all!

most likely you have a replacement screen that is missing the sleep/wake sensor. that’s why the old screen worked normally. it’s just to the left of the home button bracket if it’s there. compare the old screen to the new one and you should see the difference. or, the sensor on the new one is damaged. either way that is 1000% the problem. lots of cheap-o screens are missing the sensor.