Could it be that the track is calculated so precisely that it will read something like 332.4deg and when you select heading, it goes to 332.0 as it doesn't allow the precision of decimals. Pushing NAV it will try to recover the track again which is probably far preciser. Perhaps you can look at the IRS readout in the box and see if that gives you any clues?
I guess you could find a NAV track that is precisely aligned with a degree without a decimal and try pulling HDG for a few seconds and then pushing NAV again?