Do not delete the first .tibx file
After the very first backup version is deleted, the .tibx backup file itself will not be deleted. Instead, it will drastically reduce in size, down to 12KB or similar size (up to several MB).
This file is required for proper functioning of backup browsing mechanism.
The 12KB .tibx file will remain for as long as you keep the given backup. It will be deleted if you delete the backup along with all of its versions (backup menu option Delete - Delete entirely).