Try this. Check /etc/php7/
Rename 20_mysqli.ini to 30_mysqli.ini, or delete 20_mysqli.ini if both exist. If you also have 20_openssl.ini there, rename that to 10_openssl.ini.
Above change is needed to load the modules in the correct order.
This should now be fixed in the latest builds, but you could be running older code, or suffering from the bug that was there earlier.