Issues with putting my iPad 6 back together.

In what way did the display start acting up? You are most likely having issues with the digitizer and not the LCD itself. There are many places to get replacement parts with different quality standards so if you got something from Amazon it could be a low quality part. Did the replacement digitizer come with the cables pre-bent? Apple products are very particular about how ribbon cables are bent and this can cause issues. One other thing to look at, the frame. Is your frame bent? This can cause the digitizer not to sit right and could also cause problems.