Had a situation recently that was a bit annoying (yes, first world problems)… was flying MIA-SFO-SYD. Only 1 stop itinerary had a ~13 hour layover in SFO as the MIA-SFO nonstop gets in before 10am and UA863 isn’t until 22:55. No big deal, just work for the day in the PL after landing, step out for a meal mid-afternoon, freshen up in the PL in the evening. UA staff at MIA check-in wouldn’t accept my luggage as the system prevented “due to it being more than a 6 hour layover.” I explained the reason for the long layover (only 1 stop option). MIA staff would only check to SFO, and instructed to claim and attempt to re-check there. Claimed at SFO and attempted to check at SFO around 10am, they gave the same response indicating it was an SFO policy not to accept prior to 6 hours or anything with a 6+ hour layover. I explained the reasoning for the long layover and plan to spend the layover in the PL (and obviously can’t bring checked luggage through security). Agents were apologetic but insistent they couldn’t do anything and sent me to luggage storage. Probably something I should’ve looked into prior to planning the long layover, but still frustrating.