Even having Grandpa Simpson as a WW2 vet seems dubious now, because in the Flying Hellfish episode, his rank is shown as Master Sergeant [E-8], which meant he couldn't have been a teenager in the war; he'd have to serve for several years prior to getting that rank, and it's unlikely that he was like 17 or 18 with that rank.
After a while they didn't even care about continuity of what service or even what
theater he supposedly served in. Going from clearly in the Army in the European one (The Hellfish) to the Marine Air Service in the Pacific for a Christmas episode gag.....
It still could've been funny if it was a running gag that Grandpa can't remember exactly what he did during the war, and has picked up a false history stitched together from newsreels and other people's memories (I mostly piece it together from sugar packets) due to dementia. Or if it was played up as Grandpa being a Colonel McBragg figure who actually has fought in every war and been there at every decisive battle, winning them all unseen by the history books.....
But, the Hellfish episode develops a pretty solid continuity, giving names, places, ranks, chronology... and is as respectful of it as you can be in a comedy that's playing loose with the facts (Abe apologizes to the other deceased squad members at their graves for "Breaking" their tontine, something that wasn't even needed to advance the plot, but was deference due as their CO, showing his character) No surprise that the writers later shredded it all just for apparent want of not caring anymore.
You wonder why they don't get why continuity matters, how they can't see investing in learning someone's back story only to render it moot by pulling a Disney Star Wars/ Terminator Dark Fate retcon can make us all so upset? They don't give a fuck about their characters backstories, they're just conduits for the story and merch to flow through, what Grandpa Simpson did in WWII doesn't matter to them, why should it to you? IN fact, why should it matter what he did? He can be a WWII vet this week, a Civil War vet the next, why does it matter? right?