Macron accent above some Cyrillic letters is shifted

OpenType fonts require very patient work for supporting combining characters, that's a fact, and similar problems are, unfortunately, common.

Apparently, the font developer here forgot to add proper anchor points for combining characters in some slots of the font.

Also я Е О Э Ю Я suffer from the same problem. However, this can be cured by using the legacy method for accents.

\documentclass{scrartcl}

\usepackage{polyglossia}
\usepackage{fontspec}

\setdefaultlanguage{russian}

\setmainfont{EB Garamond}
\newfontfamily\cyrillicfont{EB Garamond}

\DeclareTextCompositeCommand{\=}{TU}{ю}{\accent"0304 ю}
\DeclareTextCompositeCommand{\=}{TU}{я}{\accent"0304 я}
\DeclareTextCompositeCommand{\=}{TU}{Е}{\accent"0304 Е}
\DeclareTextCompositeCommand{\=}{TU}{О}{\accent"0304 О}
\DeclareTextCompositeCommand{\=}{TU}{Э}{\accent"0304 Э}
\DeclareTextCompositeCommand{\=}{TU}{Ю}{\accent"0304 Ю}
\DeclareTextCompositeCommand{\=}{TU}{Я}{\accent"0304 Я}

\KOMAoptions{fontsize = 14}

\begin{document}

ш\={ю}кан

\=а\=е\=и\=о\=у\=э\=ю\=я

\=A\=Е\=И\=О\=У\=Э\=Ю\=Я

\end{document}

For the record, here's the output with the seven \DeclareTextCompositeCommand lines omitted.