That is weird. Both text field and text area field are saved as a string, so it should be compatible. Could you please check if the backend had the correct value when you changed the field type? Could you please debug it like the following?
var_dump( get_field('myfield', 99) );
Where 99 is the ID of your post/page that has the field.
Also, could you please try to create a new text area field and check if it’s working?
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