dovecot - Apr 2018 - Is FTS needed for Subject/Recipient/Sender server-side searches?

I've had a number of Dovecot installs happily running for quite a few 
years now, but I've just stumbled over a new issue. I am trying to get 
some iPhones to do server-side email searches through 
ActiveSync/Exchange protocol -> Horde -> Dovecot.

The bit I can't figure out is if I need to enable and configure FTS in 
Dovecot for that to work? I've tested server-side email searches against 
the same Dovecot server on Thunderbird - through IMAP (there is an 
option to force the search on the server in Thunderbird) - and it is 
working really well and fast. I am only really interested in 
Sender/Recipient/Subject searches.

Server-side searching is not working at the moment on the iPhones side - 
although I can see the ActiveSync commands being sent to Horde. Anyway, 
to begin with, does anybody know if server-side IMAP searches need FTS 
in Dovecot (Horde is talking to Dovecot through IMAP).

The fact that they work fine in Thunderbird would suggest the answer is 
'no' - but could someone confirm that is correct please?

Anticipated thanks for any hints. I am on Dovecot 2.2.33 if in case it 
matters.