Try

\newcommand\iddots{\mathinner{ \kern1mu\raise1pt{.} \kern2mu\raise4pt{.} \kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.} \kern1mu }} 

This is based off the actual definition of \ddots in $\rm\TeX$, which constricts the character from three separate dots like this.

Then the result for \iddots is $\newcommand\iddots{\mathinner{\kern1mu\raise1pt{.}\kern2mu\raise4pt{.}\kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}\kern1mu}}\iddots$ rather than $\unicode{x22F0}$. Compare to \ddots: $\ddots$

Try

\newcommand\iddots{\mathinner{ \kern1mu\raise1pt{.} \kern2mu\raise4pt{.} \kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.} \kern1mu }} 

This is based off the actual definition of \ddots in $\rm\TeX$, which constructs the character from three separate dots like this.

Then the result for \iddots is $\newcommand\iddots{\mathinner{\kern1mu\raise1pt{.}\kern2mu\raise4pt{.}\kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}\kern1mu}}\iddots$ rather than $\unicode{x22F0}$. Compare to \ddots: $\ddots$

Try

\newcommand\iddots{\mathinner{ \kern1mu\raise1pt{.} \kern2mu\raise4pt{.} \kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.} \kern1mu} }} 

This is based off the actual definition of \ddots in $\rm\TeX$, which constricts the character from three separate dots like this.

Then the result for \ddots is $\newcommand\iddots{\mathinner{\kern1mu\raise1pt{.}\kern2mu\raise4pt{.}\kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}\kern1mu}}\iddots$ rather than $\unicode{x22F0}$. Compare to \ddots: $\ddots$

Try

\newcommand\iddots{\mathinner{ \kern1mu\raise1pt{.} \kern2mu\raise4pt{.} \kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.} \kern1mu }} 

This is based off the actual definition of \ddots in $\rm\TeX$, which constricts the character from three separate dots like this.

Then the result for \iddots is $\newcommand\iddots{\mathinner{\kern1mu\raise1pt{.}\kern2mu\raise4pt{.}\kern2mu\raise7pt{\Rule{0pt}{7pt}{0pt}.}\kern1mu}}\iddots$ rather than $\unicode{x22F0}$. Compare to \ddots: $\ddots$