Hi Andrew,
This issue has been fixed for 3.11 as the template used for an existing feature service is built into Connect, so we can not just update the Online item. The fix will be available in the Connect 3.11 RC builds on Early Adopter Community in the next few days.
In the meantime, what you can do is take the attached internal XLSFormTemplate and copy it to the following location (on Windows):
C:\Users\{username}\AppData\Local\Temp\
This is where the internal template gets extracted to when you create a survey from an existing feature service. So if you update the template first in this location, then create the survey, it will use the new one.
Regards,
Phil.