For sub fields of any type, (Group, Repeater, Flex) you should use the field key instead of the field name I think.
Also, the 3rd parameter contains the current field array for the field being formatted. You could apply the filter to all textarea fields and then you can look at the field details and on filter specific one.
If you want to go a step further you can create your own field setting for textareas https://www.advancedcustomfields.com/resources/adding-custom-settings-fields/. Then you can filter all text area fields and check for this custom setting in the field array and do the formatting or not based on that setting.
It may also be possible to use an acf/prepare_field filter to actually modify the formatting choices given by ACF for text area fields. I’m sure this is possible because ACF uses the same filters to display its own settings fields that we can use to alter the fields we create. However, that would take a bit of digging and testing. This one is heading off into a bit of the unknown.
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