@Gunwoo Gim as a workaround, I have found that forcing it to use gcc-14 allows dkms to successfully build the zfs modules against the kernel packages from the repo. I had trouble finding a working way to configure dkms to use a specific compiler, but replacing the global /usr/bin/gcc symlink got things working.
# The gcc-14 packages likely got installed as a dependency, but double-check before replacing
ls /usr/bin/gcc-14 && sudo ln -sf /usr/bin/gcc-14 /usr/bin/gcc
This appears to confirm my theory that this is a build chain issue relating to how the CI system is building packages for the apt repo. It isn't just the kernel packages/configs that are built against gcc-14, as much of the system depends on the gcc-14-base package.
I only had a quick peek, but it looks like there was some significant work on the CI package build system between 26.5 and 26.8, and the July commits in https://github.com/armbian/ci/commits/main/userpatches strike me as suspicious in this context, but if it were just that I would expect the issue to be more widespread than it seems to be.