My 6 plus drains quickly.

I'm not aware of any battery issues with the latest iOS so that may just be a coincidence. What may be happening is that some modular element of the phone, for example a camera or sensor may be partially defective and drawing excess current, which would drain the battery quickly. This takes a while to troubleshoot because you have to run multi-hour tests to find out which one is causing this. Of course, it is also possible that the replacement battery isn't good either.

If you are comfortable, try the following:

1. Fully charge the battery
2. Close all the apps
3. Turn on max brightness and see how long the battery lasts
4. Then open up your phone, disconnect the cameras, sensors (proximity/ambient light) & the Lightning port and do the same test.

If you see the same result, then there is something wrong on the logic board itself. If you see a noticeable difference, then add one element at a time until you see what causes the drain.