In case anybody runs into the same problem (took me a day to figure out).
It's known for Formtastic and a workaround is listed here too: justinfrench/formtastic#851
Rolling back the required formtastic version to 2.0.x (if possible) may fix this problem, too.
Thanks for bringing this up. Rolling back to formtastic 2.0.x would break surveyor's supports for rails 3.21. i think the best we could do at this point is add the workaround to the README.
spork-formatastic workaround. #425
justinfrench/formtastic#851 has been closed since there is a workaround, same here.