2 step verification code interchange the last two digits

can anyone help? I have set the 2 step verification code. Sometimes, WhatsApp will prompt me to enter the code to verify. Recently, I noticed that the last two digits will inter-change. E.g. if the password is 123456. when it prompts to enter the password, 123456 doesn't work, but 123465 works. the last two digit will interchange.
could someone monitor my chats?