I always passed this off as a fluke but I've been able to reproduce it consistently on my iPhone 6 standard-sized AT&T phone. It happens when I access an item from tapping it in the Favorites list, though maybe elsewhere as well. Here's the steps to reproduce:
After the first time, it appears to open the context menu properly pretty consistently, but repeatably, the first tap on a field after opening the app causes the context menu to briefly appear and then disappear, requiring a second tap.
The reason this comes up so much is when tapping the One-Time Password field to copy the code prior to tapping the URL field to open a site, so it's ready to paste the code after the login is filled (auto-copying the code would be even better of course but I know that's not happening yet).
Note: Tapping a field like the URL works fine the first time, it's just the context menu ones. And tapping URL first doesn't fix the flashing context menu on first-tap of one of those fields, either.
Minor with an easy workaround, but still consistent and repeatable :-)
1Password Version: 6.6
Extension Version: Not Provided
OS Version: iOS 10.3.1
Sync Type: Families