On versions of Windows lower than Win10 version 1703, using "@compat=host" will cause issues with date/number formatting for a handful of locales. This is mainly due to the locales changing names between OS releases, while keeping the same LCID. (Win7 -> Win8 -> Win8.1 -> Win10).
This was actually discussed in the last TC call.
We can try to add workarounds for these few locales on down-level platforms, but the general consensus was that "@compat=host" is rarely used in practice, so we could likely have this as a "known issue" until the down-level Windows platforms are no longer supported.
Note: Both the format_number and format_date components are affected, but you can only set 1 component for a ticket.
Adding the format_date component now, since we can have more than one component in Jira.
Sorry for going tangential. I'm curious how @compat=host is implemented on platforms other than Windows. Some folks in Ecma 402 spec committee want to add that, but I've been doubtful.
AFAIK, "@compat=host" isn't implemented for any other platforms at all. It is only implemented for Windows.
I wonder if the Ecma402 interest is in the hopes that there might be some way for ICU to pick up on OS/user preferences settings (like 12/24 hour) for each OS?