httperrors existingResponse="Auto" doesn't serve custom pages

I'm trying to show custom pages in case of custom errors.

This is my configuration:

<httpErrors existingResponse="Auto" defaultResponseMode="ExecuteURL" errorMode="Custom">
    <clear/>
    <remove statusCode="403"/>
    <error statusCode="403" responseMode="ExecuteURL" path="/403.html"/>
    <remove statusCode="404"/>
    <error statusCode="404" responseMode="ExecuteURL" path="/404.html"/>
    <remove statusCode="500"/>
    <error statusCode="500" responseMode="ExecuteURL" path="/500.html"/>
</httpErrors>

It works fine for IIS errors if I use existingResponse="Replace".

But that also rejects TrySkipIisCustomErrors that is quite needy

I don't know how to fix it. If I use replace, it breaks my functionality and if I use Auto then it doesn't serve my custom pages for IIS

Any help will be appreciated