Agree that this is annoying when it is not wanted—and it has unfortunately been disrupting people ever since!
Trigger:
There is no other row in the table with a non-blank value in this field. In other words, this conversion will happen if the table is empty, or if every other row in the table has no value for this column.
Current workaround:
Have a row somewhere with a non-blank value for this column, and filter it out of view.
How to influence change:
Vote on the Text Column Type that stays text Suggestion Box entry.