How to flatten a plywood subfloor for tile?
Yes, your plan sounds perfectly fine, initially. But, that deep of a divot for such a small area is concerning to me. I'd take the plywood up or drill an inspection hole in the ceiling below to see what's going on.

To me it sounds like it could be 1 of 3 things:

1- The plywood is cracked &amp; therefore compromised, so it should be cut-out &amp; replaced in that area. But, it would be very obviously bouncy or spongy.

2- A joist was installed incorrectly &amp; is sagging down. But, the sag area is way too small.

3- The joist could be a cracked, which will only crack your new tile immediately or eventually. This may or may not reflect upon the ceiling below.

It's just very uncommon in newer construction to have such an odd deflection without a physical problem being the cause. Otherwise, fill &amp; level at your own peril.