SBC adds a record into reg-cache with registration-caching=disabled

Hello colleagues.

I faced a situation when SBC selectively adds a record into registration cache on a sip-interface where registration-caching is disabled: if I register some number from my test terminal, then it appears in reg-cache. if customer registers the same number from his terminal then SBC just forwards "reg-401-reg-200OK" and does not store the record in the cache. What can be the reason? Have you seen such the behaviour?

Thanks and regards,

Daureen