revised hepburn used for on Ninkuu => Ninkū
and the PSX version is Ninku

but not on most (if not all) of the other titles on the db.

Is it a special case because it was written as Ninkū on the cover (but also with kanji, so why not used Ninkuu like for other titles) ?
If this is the reason, why not Shienryu (written on the cover) instead of Shienryuu ?

I am for Ninkuu.

Something to check. vs smile

So the title screen is the rule ?
For all entries (SS, but also demo compilations for PSX, PS2, etc.) ?

In this case, the PSX entry for Ninku needs to be fixed.