When I was reading about bypassed garrisons, the only real details I could find were about how their offensive capabilities were dealt with (usually with airpower) and how they were left to rot afterwards. Sometimes there were attempts to resupply them, but it seems, overall, that the Japanese high command either didn't care or wasn't able to help.
With that in mind, what exactly was life like for the soldiers stranded on these islands? Also, how were they dealt with after the war?
There is an amazing but little-known documentary called (in English translation) "The Emperor's Naked Army Marches On (1988)". It is about a Japanese man, Kenzo Okuzaki, who very publicly marched to Emperor Hirohito's palace and fired a slingshot at the building in order to publicly shame the Emperor for the actions of the Japanese military leaders during WWII.
According to Okuzaki, there were instances where Japanese commanders stranded in remote Pacific locations had their own troops executed on trumped up charges, then ate their bodies. This was to fend off starvation in situations where the Japanese units were isolated with no food supplies. Okuzaki even invites himself into the homes of some of these military commanders (in the 1980's), with cameras rolling, then tells their families in front of them that their father/husband was a murdering cannibal during the war.
So to answer your question, thousands of these soldiers died of disease and starvation, cut off entirely from the rest of the Japanese forces, stranded on jungle islands, with hostile locals intent on killing them. And according to Okuzaki (and others) the result was sometimes that the officers brutalized, murdered, and committed acts of cannibalism, against their own soldiers.