The cmakev2 path snapshots SDKCONFIG_DEFAULTS once during early init. Build
system v1 apps that mutate SDKCONFIG_DEFAULTS after `include(project.cmake)`
but before `project()` (e.g. `list(PREPEND SDKCONFIG_DEFAULTS ...)` to inject
tools/test_apps/configs/sdkconfig.debug_helpers) miss those mutations when
the shim drives the cmakev2 path, because the snapshot was taken before the
mutation.
Re-resolve SDKCONFIG_DEFAULTS at sdkconfig-generation time when
__V1_COMPAT_SHIM is active so the late-mutated value is honored. Native
cmakev2 apps still use the property-based contract via
idf_build_set_property(SDKCONFIG_DEFAULTS ... APPEND).
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