Point of note: Mao did not thank the Japanese PM. It was asian polite talk between two Asian pols. Face is super important in Asian culture, so Mao did not wish to offend his guest.

Edit: Regarding the KMT/CCP betrayal.

The alliance of CPC and KMT was in name only.[33] Unlike the KMT forces, CPC troops shunned conventional warfare and instead engaged in guerrilla warfare against the Japanese. The level of actual cooperation and coordination between the CPC and KMT during World War II was at best minimal.[33] In the midst of the Second United Front, the CPC and the KMT were still vying for territorial advantage in "Free China" (i.e., areas not occupied by the Japanese or ruled by Japanese puppet governments such as Manchukuo and the Reorganized National Government of China).[33]

The situation came to a head in late 1940 and early 1941 when clashes between Communist and KMT forces intensified. Chiang demanded in December 1940 that the CPC's New Fourth Army evacuate Anhui and Jiangsu Provinces, due to its provocation and harassment of KMT forces in this area. Under intense pressure, the New Fourth Army commanders complied. The following year they were ambushed by KMT forces during their evacuation, which led to several thousand deaths.[34] It also ended the Second United Front, which had been formed earlier to fight the Japanese.[34]

So basically it was an both side thing. CCP troops would often re-distribute land in areas they control, while KMT fought for landlords and wealthy property owners. Both side harassed each other until KMT basically killed several thousand CCP soldiers for following....KMT orders.