It’s more than just one seed, I believe that it is all of them and that it doesn’t just stop at strongholds, but other structures as well. Sometimes their structure location will be there, just not the structure itself.
Seed Map - Minecraft App Example 1
Seed Map - Minecraft App Example 2
Seed Map - Minecraft App Example 3
Seed Map - Minecraft App Example 4
I also reviewed it in the game on the latest snapshot and teleported further than I stated the structure stop spawning to test my theory, which was ultimately proven correct when I did /locate structure.
It’s not just the positive coordinates, but the negative one’s too.
I would like to add that I suspect it may be also that they did not originally close them that it may or may not cause them to close them again.