Does flashing stock (Sprint) ROM relock it?

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I see all these posts of trouble relocking.

I had unlocked it.

Tried Might ROM (but before doing so put the early Leaked ROM on internal storage as backup).

Decided to go to the Leaked ROM and loaded it from doing the magic dance (vol down, power, hangup, reset) and that all worked fine.

So I wanted to update my backup on internal storage I put the new official ROM on the internal storage and the magic dance no longer worked? It appears to be relocked again.

So I updated it via active sync for now.

So if it relocks it so easily why are some folks having issues relocking?