[exim] Clarification on detainting in exim 4.94 router

Hello,

I have the following router and transport for delivery to a ticketing
system. There's an issue here with $local_part. Isn't it detainted by
the use of local_parts to only run this if $local_part was found in the
file? I'd appreciate some advice on the correct way to do this in exim
4.94. $local_part_data is blank.

# router
request_tracker:
   driver            = redirect
   domains           = rt.example.com
   local_parts       = /usr/local/etc/exim/queuenames/list.of.requestors
   local_part_suffix = -comment
   local_part_suffix_optional
   pipe_transport    = request_tracker_pipe
   data              =   "|/usr/local/bin/rt-mailgate \
                         --queue \"${local_part}\" \
                         --action ${substr_1:${if 
eq{$local_part_suffix}{} \
                         {-correspond}{$local_part_suffix}} } \
                         --url http://rt.example.com"
   user              = mailnull

# transport
request_tracker_pipe:
   driver         = pipe
   return_fail_output
   allow_commands = /usr/local/bin/rt-mailgate

Thanks,
Adam