I'm in a somewhat similar situation in that I have a morning flight SLC-ICN in D1, but am flying LAX-SLC the night before (same PNR), with an overnight connection. I had assumed no D1 Lounge entry at LAX due to the entry requirements specifying "same day" D1 boarding pass.

As a data point, I was already checked in online for my following day SLC-ICN D1 flight, presented my D1 boarding pass at the LAX D1 Check-in, and was granted access after it was scanned.

Same with me. Flew domestic to JFK in MC one afternoon and had a next-day flight in UC on Virgin. Showed them the Virgin bp and was granted access. Different PNR.

Anyone tried accessing the D1 lounge with the long haul flight the day BEFORE the domestic connection (but within 24 hours)?

This was separate tickets so YMMV if it's on a single booking, but got denied the next morning at LAX when flying BOS-LAX (D1, 11pm arrival) followed by LAX-SFO (6:30am departure the next day) - showed the BOS-LAX BP which they scanned but "computer says no"

Another answer to my own question - but it could have been the arrival class that made the difference.

Arrived in AF P 21:00 day 1
Departed in WS J 17:30 day 2

Lounge dragon (who was very friendly) overrode the "computer says no" when scanning the AF boarding pass and allowed us in.

100% right. But think that if the earlier suggested 2000 passengers per day DP is accurate or close to, the person scanning you in has probably seen 100, 200 maybe even 300 people who are granted access by their scanner and perhaps 50-100 or so more who try but are denied. Point being, we are probably just another face, and barcode. We look at our BPs and it is probably the only one we look at, are focused on. The lounge staff, generally speaking are nowhere near as focused on our BP as we are. That magic purple definitely makes their life easier I am sure. As to why it sometimes shows up in app and not is another one of the Delta IT mysteries.

and of course there are plenty of scenarios where one might be flying e.g. JFK-ATL-HND where the boarding pass you're scanning at JFK to access the lounge isn't a "D1" boarding pass, yet you're entitled to access anyway.