App Version (from "Chrome Settings > About Chrome"): M54 stable, M56 canary
iOS Version: 10.1
Device: 6S

Steps to reproduce: 
1. Navigate to news.ycombinator.com
2. Click on one of the top links.
3. Browse around other links on the page (optional).  
You need to wait for a period of time (not sure how long). I've seen it happen without
navigating to other pages (just news.ycombinator.com > one page > hit back).
4. Press the back button.

Observed behavior: 
You see an old snapshot of the page.

Expected behavior: 
You should see the same page in (1) or an updated version of it.

Frequency: Hard to say. It's definitely reproducible (have seen it on multiple phones),
but due to the time duration it doesn't happen super frequently. 

Additional comments: 
* Issue also seen in App Store reviews (see attached screenshot).
* For test sites, either news.ycombinator.com or reddit.com are good to test because
the page content changes frequently.
* You probably have to have visited the page previously for it to be cached.