[Exim] Change in working of message_filter

Hi,
I recently upgraded one of my machines from 3.12 to 3.15 (Solaris 2.5.1).
While testing
I noticed that a message (frozen by message_filter) I was trying to deliver
forceably using
EXIMON (Deliver message) was being discarded with the message:

Unfrozen by forced delivery
=> discarded (message_filter)

The basics of the filter are:

# Exim filter
#
if error_message or not first_delivery
then
finish
endif

logfile /usr/exim/spool/log/system-filterlog

if ......
then
logwrite ....
freeze text "....
.....
.....
seen finish
endif

This works as I expect (i.e. it delivers the message) in 3.12 and 3.14 but
gives the above error
using 3.15. Also (3.15) auto_thawing the message works as normal but
forceably thawing the
message (again via EXIMON) and letting a queue run pick it up, again
discards the message.

Has something changed in 3.15 that would produce this effect? I did look at
the ChangeLog and
NewStuff but....

regards
Martyn Griffiths
Network Service Group
University of Glamorgan