I have no idea which expi date would be correct. I’d shoot an email to White Labs. A possible explanation on the difference could be due to FDA regs if packets of Lacto are included in such a thing. FDA might require 1 timeframe for the package while White Labs might suggest a 2nd date (based on something else, maybe research) with the caveat that it is intended for brewing and not immediate consumption. Or maybe the stamper just wasn’t set correctly that day. Or maybe Fred hit 3 instead of 1 on the computer. This is all speculation and I’d love to hear what White Labs would say.
I would still just use that packet as-is for your brew day. I know that slurry has a lot of Lacto in it compared to a packet but I have used small amounts of very old slurry, that set at room temp to make sour bombs.
+1 on reading Fast Souring if you haven’t already.