This caused it, not integrating updates will provide a 24H2 ISO, 26100.1.. The option to integrate updates is selected by default for a reason, without it selected you will always end-up with the base ISO.
When I downloaded Windows 11 version 26H1 (28000.2956) amd64 I did not include updates, yet I properly got 26H1. Even it worked for Windows 11 version 26H2 (26300.9457) amd64 so it seems something went wrong for 9539. However this 26H2 does not seem correct, it is 26300. I will still recheck including updates. Thanks.
Same logic does not apply across all builds. 26H1 is only one base build for the time being. It doesn't have various enablement packages like 24H2/25H2/26H2, including updates or not will always result in build 28000.x. 25H2 and 26H2 are both 24H2 26100.x with enablement package applied, they aren't actual new builds. I'm more inclined to think you forgot to exclude updates, so you got the desired iso as intended. The mechanism is the same - build numbers are dependent on updates. If you don't include them, you will get the base build 2xxxx.1.
Because that is not the same as 24/25/26H2, it is an entirely new major build = 28000. 26100/26200/26220/26300/26340 all are based on 26000.x.