We found out that our win32 stdatomic.h shim converts all signed integers to unsigned. The proper workaround for this is to load the value into a local variable first and then use the variable instead of directly calling atomic_load(). We need to investigate the rest of the atomic_int_* usage in BIND 9 source code for similar errors:

lib/isc/hp.c:static atomic_int_fast32_t tid_v_base = ATOMIC_VAR_INIT(0);
lib/isc/include/isc/mutexatomic.h:typedef struct atomic_int_fast32 {
lib/isc/include/isc/mutexatomic.h:} atomic_int_fast32_t;
lib/isc/include/isc/mutexatomic.h:typedef struct atomic_int_fast64 {
lib/isc/include/isc/mutexatomic.h:} atomic_int_fast64_t;
lib/isc/include/isc/rwlock.h:   atomic_int_fast32_t spins;
lib/isc/include/isc/rwlock.h:   atomic_int_fast32_t write_requests;
lib/isc/include/isc/rwlock.h:   atomic_int_fast32_t write_completions;
lib/isc/include/isc/rwlock.h:   atomic_int_fast32_t cnt_and_flag;
lib/isc/log.c:  atomic_int_fast32_t debug_level;
lib/isc/log.c:  atomic_int_fast32_t highest_level;
lib/isc/netmgr/netmgr-int.h:    atomic_int_fast64_t pktcount;
lib/isc/netmgr/netmgr-int.h:    atomic_int_fast32_t rchildren;
lib/isc/netmgr/netmgr-int.h:    atomic_int_fast32_t ah;
lib/isc/stats.c:typedef atomic_int_fast32_t isc__atomic_statcounter_t;
lib/isc/stats.c:typedef atomic_int_fast64_t isc__atomic_statcounter_t;
lib/isc/tests/task_test.c:atomic_int_fast32_t counter;
lib/isc/tests/task_test.c:      atomic_int_fast32_t *value = (atomic_int_fast32_t *)event->ev_arg;
lib/isc/tests/task_test.c:      atomic_int_fast32_t *value = (atomic_int_fast32_t *)event->ev_arg;
lib/isc/tests/task_test.c:      atomic_int_fast32_t a, b;
lib/isc/tests/task_test.c:      atomic_int_fast32_t a, b, c, d, e;
lib/isc/tests/task_test.c:      atomic_int_fast32_t a, b, c, d, e; /* non valid states */
lib/isc/tests/timer_test.c:static atomic_int_fast32_t eventcnt;