When dealing with a print white ink in say packaging, it is generally necessary to place it above all other artwork (CMYK or spot) so that the other content does not knock out unwanted holes into the spot white. Print white not set to overprint or multiply will of course knock out and remove the content below, so the spot white is then set to overprint so that underlying artwork is not adversely affected. This is wanted and by design for spot print white ink.
If there is no print white separation required, then there are less common cases where an overprint white is intended, in most cases it would be a mistake.
Probably best to flag/warn this in preflight without changing the setting if this is a concern. Many prepress workflow systems and RIPs are set to change overprinting whites, which is a safe assumption – except for when it isn’t :]
Stephen Marsh