LiquidCrystal I2C with multiple LCDs : LCDs stagnate after custom caracters creation

Yes.
I created & support the hd44780 library.
It is at least 2x the speed, and has many additional features and lots of documentation that comes with it.
There is even a wiki.

There is nobody supporting the LiquidCrystal_I2C library, not to mention that there are multiple "LiquidCrystal_I2C" libraries floating around out there and they are not all the same.

The LiquidCrystal_I2C library in the IDE library manager, was taken over by a guy name John Rickman who made a wreck of it.
Here is more about that:

--- bill