Just found that this is what there is in the / directory, in php.ini:
extension=memcached.so
extension=memcache.so
...
session.save_handler = memcache
session.save_path = "tcp://10.168.1.55:11211?persistent=0&weight=1&timeo
The date on php.ini is 25/7/2017, a year before I changed to php 7.1, and it did not have any effect when running php 5.6.
Could this be it? Php overriding the setting in CI config.sys?