It is always annoying when not getting credit for a factor found. There is of course no way to prevent sieving finding factors in your factoring range. It is only possible check for found factors before starting factoring. The factor you post is 13.8P which is in a sieve range PrimeGrid did a some time ago and when I look in the "worktodo_16000000_16500000.zip" posted in the "P1 coordination thread", the test you post have been excluded. You should use an updated worktodo file.