Another font is overriding MingLiUExtB, or you are using software that does not support Plane 2 (e.g., old Adobe Acrobat 9).
While MingLiU-ExtB is widely used, there are some free and open-source alternatives: mingliuextb font
While excellent for legibility, the serif design may feel outdated for modern web or graphic design compared to san-serif options like Noto Sans CJK . Another font is overriding MingLiUExtB, or you are
Without this font, a character like 𠀀 (U+20000—an ancient variation of "one") will show as a blank box. Another font is overriding MingLiUExtB