wtf? forgot? :*: i can undertsand if they had pressing matters to attend to not immediately take over but why didn't they try to take it back later?
i understand mao zadong annexed a large portion of shan state land near burma where the nationalists were hiding to kick them out of china.
the Communist was chasing the Nationalist to inland Chengdu and ignored/forgot the border areas. maybe the CCP thought that those countries would not take control of the border areas. at that time in late 1950, the PRC had ruled China for only a year. they had lots of pressing maters to attend to and solve. the Nationalist still held up in Hanan island till May 1950 then the Korean war happened. very few countries recognized the PRC. the PRC was not a United Nations member. they can't complain about the border intrusions to the United Nations.
i think the PRC was reluctant in the first few years of its establishment to solve the border issues. there were many more important matters to deal with, internally and externally. the PRC did not want to weaken the relations with the few countries that recognized it internationally like India and Burma. so PRC allowed India to have its way with the border issues and gave Shan state to Burma.