The G1HeapRegionSize flag is not updated with the actual value of the region size. This makes it impossible to figure out the heap region size using PrintFlagsFinal:

$ java -XX:+UseG1GC -XX:+PrintFlagsFinal -version | grep G1HeapRegionSize
    uintx G1HeapRegionSize = 0 {product}

Or when setting it to the wrong value (this will actually use a region size of 8m):

$ java -XX:+UseG1GC -XX:G1HeapRegionSize=11m -XX:+PrintFlagsFinal -version | grep G1HeapRegionSize
    uintx G1HeapRegionSize := 11534336 {product}