Hail mary.

tools/bazel.rc

@@ -51,6 +51,10 @@ build:ubsan --copt=-DUNDEFINED_BEHAVIOR_SANITIZER  # used by absl
 build:ubsan --copt=-fno-sanitize=function,vptr
 build:ubsan --linkopt=-fsanitize=undefined
 build:ubsan --action_env=UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1:suppressions=test/core/util/ubsan_suppressions.txt
+# For some reasons, these two stopped being propagated, so, redeclaring them here.
+# That's a hack that needs to be removed once we understand what's going on.
+build:ubsan --copt=-DPB_FIELD_32BIT=1
+build:ubsan --copt=-DGRPC_PORT_ISOLATED_RUNTIME=1
 
 build:basicprof --strip=never
 build:basicprof --copt=-DNDEBUG