Currently, Fivetran maps the Decimal columns from MYSQL to DECIMAL on bigquery side. However, bigquery does not support precisions for decimal that is more than 9. This means that any column from the source that has a precision of more than 9 will be mapped to FLOAT64 on bigquery side rather than DECIMAL.
This is specifically happening for columns that would store latitude and longitude information.
Ideally, it should be mapped to BIGDECIMAL on bigquery side that will support larger precision values.
Related support ticket: 25740