The partition table of the created image is:
Device Boot Start End Sectors Size Id Type
pi3-test.img1 * 2048 264191 262144 128M c W95 FAT32 (LBA)
pi3-test.img2 264192 1210367 946176 462M 83 Linux
After I re-create the 2nd partition to be like this, I can then mount it:
Device Boot Start End Sectors Size Id Type
pi3-test.img1 * 2048 264191 262144 128M c W95 FAT32 (LBA)
pi3-test.img2 264192 1212171 947980 462.9M 83 Linux
Looks like the partition table is incorrect.
The partition table of the created image is:
Device Boot Start End Sectors Size Id Type
pi3-test.img1 * 2048 264191 262144 128M c W95 FAT32 (LBA)
pi3-test.img2 264192 1210367 946176 462M 83 Linux
After I re-create the 2nd partition to be like this, I can then mount it:
Device Boot Start End Sectors Size Id Type
pi3-test.img1 * 2048 264191 262144 128M c W95 FAT32 (LBA)
pi3-test.img2 264192 1212171 947980 462.9M 83 Linux