If you showed me a community of Bangladeshis who behaved exactly like Japanese people but weren’t accepted

Koreans fit that bill; there's a lot of ethnically Korean people whose families have been in Japan multiple generations (not just assimilated as first-gen immigrants) but there's still significant social prejudice against them; life people of Chinese/Vietnamese/Thai descent who've grown up in Japan as opposed to being recent arrivals. If you haunt Japanese nationalist online communities you can see this on a daily basis.