Who did the installation? A lot of installer hold the tank by the Douglas valve or the ball cock when lowering the tank to the bowl.
You may also have a not so good brand you have been using.
Crane toilets I never had a problem with with over 1,500 installed up to date.
The Crane toilet cost around $59.00 for the 1.6 and it is worth looking into. I have installed these in both my offices and never had a problem. Good luck