One solution is to disable the 'Click Icons in Field'. While that will resolve the issue, it will also force your to go through the LastPass icon in your browser's toolbar to select a different login if the one that has been auto-filled isn't the one you want to use.
To disable 'Click Icons in Field':
- Click 'Preferences' from the LastPass menu in your browser's toolbar
- Go to 'Notifications'
- Un-check 'Click Icons in Field'
Source of the solution: https://forums.lastpass.com/viewtopic.php?f=7&t=104305#p357065