Weird Database Encryption Bug

We are currently migrating databases from MySQL to
Postgres SQL when we noticed this weird bug. In the
database we encrypted the ID numbers of the users.
Now whenever a transaction is requested by a certain
user we log the transaction by getting the encrypted
ID number, time, etc and put it in a history table.

For example the encrypted ID number is something like
12345abcdef in the users table. The input for the ID
number in the history table became something like
12345ghiklm. Now we found this strange since we just
copy the ID number from the users table to the history
table.

Any thoughts on why this happened? Can this kind of
bug happen in Postgres SQL as well?

Regards,
Joanne

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