I'm not sure this is UA's fault, but it's definitely a problem.

The UA to LX interface delay can also affect code-share bookings on United that involve LX metal. I had a situation earlier this year when a ZRH-MXP code share put my entire UA reservation in limbo for 24 hours.

If they'd rejected the booking at the LX end, my fare protection would have vanished. Also, while the booking is in limbo, you can't apply GPUs to upgrade UA flights with R space. Again, I was lucky that the R space didn't disappear over 24 hours, as I would then have had to play the waitlist lottery.

This doesn't happen on LH, where code-share and other interline bookings go through instantly. So I'm guessing it's a problem at the LX end.