Thank you for your bug report.
Currently, our developers are working on the polishing the Apex Code Assistance functionality. The issue you are faced with will be fixed in the frame of this work.
As a workaround, you can disable the preselection option for the Code Completion suggestions. You can find this using the following way: Main Menu -> Tools -> Options -> Text Editor -> Code Assistance; please disable the 'Preselect Completions in Completion List' in the Code Completion section and click 'Ok'.
I kindly ask you to let us know if the issue is reproduced for you after using this workaround by you.