Blob binary data is wrong

My table(utf-8) has a BLOB column which named mydata.

SELECT mydata, hex(mydata) FROM mytable;

The result of mydata and hex(mydata) is different.

0x789CAB3F000175003F, 0x789CABAE0500017500F9

Some data was replaced by 3F.

Is this bug? or normal?

I want to see or edit in 'Data' tab directly.

Help me plz.

Thank you.