converting username to lowercase

[JAHANZAIB SYED]

I am running FreeRADIUS Version 2.2.8 (yes its old) along with Mikrotik Router as NAS (pppoe Server)
in radacct table see some user entries with mixed uppercase ,example

User1
USER2

in radcheck, the user is defined in lower case. why radacct is putting some user with random lower/uppercase ?
How can I force it to use only lowercase for usernames?
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