The use of seppuku started declining even as early as Edo era. Even then it was used as a capital punishment, not something done truly out of 'honour'. It had declined so much that by the end of Edo era the person only mimicked the cutting motion with a folding fan and the actual death was delivered by a close friend (a kaishaku).

With that said though, a few officers did attempt seppuku with varying success, a notable case being that of Dakajiro Onishi, the brain behind the blunder that was Kamikaze.

What he did was he committed a hara kiri without a kaishaku. Seppuku is a very messy way to go. It doesn't kill you instantly so a close friend called Kaishaku was entrusted with decapitating the man once he finised the motion. Well, Onishi didn't have that so depending on the sources he was writhing for 15 hours before he died.

So TL;DR- Hara kiri was largely ceremonial by WW2 and while some did kill themselves with it, many did not.

I only know of the case of General Usjijima and Luitenant-General Cho in the last stages of the Battle of Okinawa. They performed a hari kari when they were surrounded in their armed bunker by the Americans. They ripped up their own belly and were beheaded at the same time.

Source : Anthony Beevor, 'The Second World War'