Slitherine

It's a shame bunkers can't be abandoned. At least I've not figured out how. I had to retreat at Bataan, leaving whoever was inside the bunkers to just fight way until they're destroyed...

I'm not too concerned. Bunkers and MGs are quite cheap to build by heavy and regular infantry and you get to keep the original units.
I think they are the best defensive weapons in the Japanese tool-box.
Converting them back to a new infantry unit and have this escape is too much of an advantage IMO.
After, they'll would need to leave the structure anyway.

Bunkers and pillboxes are manned by relatively few troops, no more than a couple of machine guns and light artillery pieces with a few guards, at most (certainly less than 50, often no more than a small machine gun team, say 5)). Now it's likely each infantry unit in OoB is a company, battalion, regiment, or perhaps even a division depending on the scale of the scenario. Being able to convert bunkers and pillboxes into a company (200 or so men), battalion (800), regiment (2/3000) would be an unrealistic event.

It's not really an issue. I'd think in the real war however the guys inside bunkers were allowed to retreat and get out rather than all being killed. Oh well...

Think of it as only the physical structure of the bunkers are destroyed. The troops inside retreat to be used as elite replacements.

Elite replacements!?! Most bunker troops were about as far from elite as you can get. They tended to be green recruits, overaged home guard types, ofttimes physically unfit, in other words, those not qualified to be real combat troops.The Germans used lots of Osttruppen, usually Russian prisoners of war pressed into use as guard troops, in Normandy. Volkssturm, mostly overaged men and kids were used late in the war to stem the tide, with predictable results. Cannon fodder replacements more likely.

So, more like Sgt Schultz....

Which explains your bunker icon, apparently.