mail filter for tabs has no effect

Mail using the add_filter('cf7sg_mailtag_grid_fields','insert_table_in_mail',10,5);
filter is the exact same as mail without the filter.

Email is formatted as:

Field Name
tab(1): value
tab(2): value
tab(3): value

Field Name
tab(1): value
tab(2): value
tab(3): value

Where “Field Name” and “value” are the actual field names and values, but “tab(1):” “tab(2):” “tab(3):” show up exactly as quoted, with no filter css rules having been applied to the email.

I did make sure the form key is the correct name in the filter function. The filter has no effect at all on mail output.
Same result with different themes, including vanilla 2019,2020, etc.