Tabularx X column doing the opposite of expected and becoming incredibly narrow

If you have

documentclass{article}

usepackage[table]{xcolor}
usepackage{tabularx}

begin{document}

noindent
begin{tabularx}{textwidth}{|l|X|l|l|l|}
hline
 &   & multicolumn{3}{c|}{textbf{TEST}}hline
 textbf{ID} & textbf{CRITERIA}  & textbf{(A)}  & textbf{(B)}   & textbf{(C)}  hline
 multicolumn{5}{|c|}{cellcolor[HTML]{FFFFFF}{color[HTML]{000000} textbf{Category}}}  hline
 #1 & Very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very long text
 &  Not so long text & Bit longer text than was before, but still short & Usually empty  hline
end{tabularx}
end{document}

then you are specifying that 4 of the columns are set in left aligned single column cells at the natural width of the largest entry in that column. After those widths are determined TeX will then try to make the total width of the table the specified textwidth by adjusting the width of the X columns, and allowing lien breaking within those cells.

You gave no indication of the text width (or how cellcolor was defined) but assuming a standard article and xcolor then the above example produces

You could replace l by L as you suggest in the question or for a hand written document table (as opposed to an automated database dump) it is often better to choose the column widths manualy using p{4cm} or whatever width you want and then just leave one column X to fill out the remaining space if you want the table full width.

documentclass{article}

usepackage[table]{xcolor}
usepackage{tabularx}

begin{document}

begin{center}
setlengthextrarowheight{2pt}
begin{tabularx}{textwidth}{|l|X|>{raggedright}p{2cm}|>{raggedright}p{3cm}|l|}
hline
 &   & multicolumn{3}{|c|}{textbf{TEST}}hline
 textbf{ID} & textbf{CRITERIA}  & textbf{(A)}  & textbf{(B)}   & textbf{(C)}  hline
 multicolumn{5}{|c|}{cellcolor[HTML]{FFFFFF}{color[HTML]{000000} textbf{Category}}}  hline
 #1 & Very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very long text
 &  Not so long text & Bit longer text than was before, but still short & Usually empty  hline
end{tabularx}
 
end{center}

end{document}