i have just tested this with a basic record containing your serialized data, e.g. the 2 line csv file:
In this exampe the serialized data was imported into the main post content and into a custom field which did not double serialize the data.
Based on you mentioning you are imporitng it into a "taxonomy" field type, i guessing this is the acf taxonomy field type?
Due to the way acf saves data you would have to bypass the acf saving with your raw serialized data for example.
To do this you need to get the acf field key and field name, this can be done when viewing an acf field group, via custom fields > Fields groups, when viewing a field group, under the screen options tab at the top enable the field keys option.
Now remove the original custom field on the importwp importer edit screen, and add two new custom fields, checking the enable text field checkbox to allow manual input, using the previous field name and field key values found add the two custom fields as follows (replacing field_name and field_key) with values:
custom field 1:
value: YOUR SERIALIZED DATA
custom field 2: prefix name with _