Try parsing description instead

cmake/modules/VersionInfo.cmake

@@ -8,15 +8,17 @@ IF(GIT_FOUND AND NOT FORCE_VERSION)
 	# number from the environment and add it to the build metadata
 	if("$ENV{GITHUB_REF}" MATCHES "refs/pull/([0-9]+)/merge")
 		list(APPEND BUILD_METADATA "pr${CMAKE_MATCH_1}")
+		# Parse hash from merge description
+		# e.g. "Merge abc1234 into def4567"
 		execute_process(
-			COMMAND "${GIT_EXECUTABLE}" log --format=%h HEAD~1...HEAD~2
+			COMMAND "${GIT_EXECUTABLE}" git log -n 1 --format=%s
 			OUTPUT_VARIABLE COMMIT_HASH
 			WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
 			TIMEOUT 10
 			OUTPUT_STRIP_TRAILING_WHITESPACE)
 		# If successful, use the first 7 characters to mimic github's hash style
 		if(COMMIT_HASH)
-			string(SUBSTRING "${COMMIT_HASH}" 0 7 COMMIT_HASH)
+			string(SUBSTRING "${COMMIT_HASH}" 7 7 COMMIT_HASH)
 		endif()
 	endif()