[exim] Filters: "seen finish" apparently not working

I have a .forward Exim filter file which (inter alia) contains the following statement:

if $h_X-Oxmail-Spam-Level: contains "********"
then
seen finish
endif

despite which emails with (for example) X-Oxmail-Spam-Level: ***********************

which are correctly interpreted by the filter in verbose test mode (exim -bf -v .forward <email6.txt):

____________________

Testing Exim filter file ".forward"

Condition is false: error_message

<snip>

Condition is true: $h_X-Oxmail-Spam-Level: contains ********
Seen finish
Filtering set up at least one significant delivery or other action.
No other deliveries will occur.

_____________________

are still being delivered to my inbox rather than the promised 'black hole'.

In all other respects the (quite detailed) filter operates correctly.

Am I missing something? I am running the test(s) on a text copies of the 'successfully' delivered email(s). Is it possible that the headers post delivery are different from those encountered during normal processing of the .forward filter in some way that alters its interpretation?

I am happy to share the full filter/test emails privately with anyone who can help.