Hard to say if it is a bug or intended behaviour (a feature) without more details.
Can you reproduce the behaviour at will?
Did it not display the "Tom" contact at all, when you typed 't' or did it display something else in addition (and did those additional contacts have a 't' elsewhere in the name field(s))?
My Nokia 7610 (which I bought 4 months ago) is now sent for repair, some of the keys are not functioning. Will update you again once I got the phone back.