Trouble with Bangla Font Weight

This is certainly a font problem. If you examine the files SutonnyMJ-*.ttf, then you see that they are of sizes around 73 KB, while SutonnyOMJ.ttf is about 352 KB! This should raise a red flag already.

Indeed, the fonts SutonnyMJ-*.ttf:

1. Do not follow the Unicode standard to assign glyph to the correct code point. For example, the glyph for U+0995 (Bengali Letter Ka) is encoded at U+004B (Latin Capital Letter K).
2. Do not contain the necessary lookup table(s) to perform consonant cluster ligatures. For example, <U+09B8 U+09CD U+09A5> (Bengali Letter Sa, Bengali Sign Virama, Bengali Letter Tha) cannot form the correct cluster.

\documentclass{article}
\usepackage{fontspec}
\setmainfont{SutonnyOMJ}[
    Extension = .ttf,
    BoldFont = SutonnyMJ-Bold,
    ItalicFont = SutonnyMJ-Italic,
    BoldItalicFont = SutonnyMJ-BoldItalic,
%    RawFeature = {mode=harf}
]
\parindent 0pt
\begin{document}

    কি অবস্থা? \textbf{wK Ae¯’v?} \textit{wK Ae¯’v?} \\
    \textbf{\textit{wK Ae¯’v?}} \emph{wK Ae¯’v?}

\medskip

\char"0995\char"09BF\ \char"0985\char"09AC\char"09B8\char"09CD\char"09A5\char"09BE\char"003F \\
\char"0077\char"004B\ \char"0041\char"0065\char"00AF\char"2019\char"0076\char"003F\ %
\textbf{\char"0077\char"004B\ \char"0041\char"0065\char"00AF\char"2019\char"0076\char"003F}

\end{document}

The fonts are broken, the italic, bold and bold italic fonts have the Bangala glyphs in the place of Latin characters. Only the regular font is correctly encoded: