Something in overwatch was not set to epic, or scaling was not 1440p, but rather 1080p and you didn't notice. The scaling takes a huge amount of resources. 1080TI even with 960 pro, 7700k, and 32gb dual rank 3200 cl14 is incapable of maintaining above 144fps at 1440p with correct scaling and all epic settings even when cpu and graphics are OC to max. There will always be drops below 144. If you were getting steady 190 at 1440p before, something was set incorrectly. You might get 190 at MAX FPS, but not average or min. Average should break 144, managing about 155...but minimums will drop below 144 even with 1080TI. My guess is scaling was incorrect even though you had pure epic settings and you just didn't notice.
Minimum 1% with the setup I mentioned above is only 125-130fps. With max OC on that setup, it would reach around 135-140. You have 6700k, and did not mention OC or ram speed, but simply the amount of ram...your 1% minimum is probably around 120-125 if the ram is fast enough. If you have default ram speeds, that may reach closer to 115 minimum, possibly even 110.
Overwatch occasionally alters Scaling after big patches. I've had to fix it several times before. So my guess is incorrect scaling was the issue.
So yes, hate to rain on your parade, but not even 1080ti or Titan XP is capable of maintaining above 144 with max settings at 1440p on Overwatch.