%with hyperref is loaded, this produces a bad .idx entry
comma@parse{But this fails on makeindex|textbf}{index{comma@entry}@gobble}

indexentry{But this fails on makeindex|textbf|hyperpage}{1}

documentclass[12pt]{book}

usepackage{makeidx}makeindex
usepackage{lipsum}
usepackage{kvsetkeys}
usepackage{hyperref}

begin{document}

lipsum[1]

index{This works fine|textbf}
makeatletter
    comma@parse{So does this}{index{comma@entry}@gobble}
    comma@parse{But this fails on makeindex|textbf}{index{comma@entry}@gobble}
makeatother

printindex

end{document}

indexentry{This works fine|hyperindexformat{textbf}}{1}
indexentry{So does this|hyperpage}{1}
indexentry{But this fails on makeindex|textbf|hyperpage}{1}