Odd issue I've been able to reproduce. I have multiple Google accounts, each with two-auth turned on. When I try to log into the one at the top of the list (filtered for Google) the two-auth code copies with no issue. If I try to log into one a few entries lower, copying does nothing. I can reproduce this every time no matter which account is at the top vs. lower in the list.
1Password Version: 7.3.2.BETA-0
Extension Version: Not Provided
OS Version: 10.14 build 18G59b
Sync Type: Not Provided