MD_Parola display issue when constructing test
The library documentation states:

The library does not allocate any memory for the text message

That suggests, that the buffer has to be valid beyond the execution of the displayText() function. But as you declare it inside of your updateTime() function, it the buffer gets out of scope, when exiting that function. The memory space can then be reused by the rest of the program. At that point the library will still have the pointer to the old location, though that pointer can now be considered invalid. Thus you are just reading, what the compiler decided to place in that memory location.
To solve that, you need to keep the buffer variable in scope. You can do this by moving the declaration to the global scope.