my son (7 weeks) also has this problem, he started to drink 和光堂細仔 since 2 week old and the 粒粒 started on his face from the 4th week onwards.
My "pui yuet" said that he's not 奶粉敏感 and it should be due to my "hot" food during breast feed and the hot weather. I decided to ask my doctor during his injection as long as he did not "拗".