OP, make certain that the TA is giving you the same LH connection FRA/IST when using the earlier UA flight. Since I was curious about this, I picked 2 random dates (12/10-12/14), and got interesting results:
UA8828/UA9126 (both LH), the 2:50 dep, priced at $1343 (H/H).
UA903/UA9126, (UA/LH) the 1:50 dep, priced at $1343 (H/H).
but
UA903/TK1588, also the 1:50 dep but dep IST 1 hr earlier than LH, priced at $3595 (Y/B) -- guessing this may be the "married" condition based on the fare classes.
So, maybe when they priced using UA903, they picked the TK flight which became the earliest connection.