I can't reproduce this.
Please use the official build for reporting/screenshots.
And attach a simple .blend file.
And of course try loading the default settings.

Yes, this is in object mode. So it Blender can't mirror across the global X axis because of what you say (if that is indeed what you mean), then what is the purpose of "Mirror X Global"?

Sample file - as I said, this is a ridiculously simple problem, so the blend is probably useless anyway. ;)

@Sean Robertson (seanr) As @Germano Cavalcante (mano-wii) said, Mirror > Global X just inverts the sign of the X-axis scale of the object, when used in object mode. It does not take the rotation into account. I can see that this is confusing, but this is how the operator works at least since 2.79 if not longer.

In 2.79, in Object Mode, in the Mirror menu was only Global's, (should be Local though).

https://developer.blender.org/rBf889050aea70c2ff3c36c8c8600dd2b02d5e9f91

I think in the Object Mode the Global's should be hidden, (there's still interactive mode and redo).

I can propose a patch if it's reasonable.

So this may be "as designed", but how it's designed is illogical - mirroring should be moving all vertices to the opposite side of the axis chosen (just invert the distance of each vertex from that axis). If it isn't doing that, it's not really mirroring. ;) Further, there clearly _aren't_ two different operations given that it isn't really mirroring in that sense, so why two different buttons?

@Sean Robertson (seanr) I completely agree that how it's currently implemented and how it's named doesn't match. @Yevgeny Makarov (jenkm) I think this would be a good idea, but even better if the operator does what its name says (in object mode). Just for clarification, I had to close the ticket because we have fairly strict rules when it comes to what is considered a bug and I will have to follow the guidance of our core developers.

@Germano Cavalcante (mano-wii) If you think this should be reopened and marked as "ToDo" or confirmed as known issue, let me know.

@Robert Guetzkow (rjg) if it's not appropriate here per those rules, then where is it appropriate? It may not technically be a bug from the developers' perspective, but it's absolutely horrible behavior that from the perspective of the user is a bug, even if the bug is the design itself. There's a lot of UX stuff like this in Blender, frankly, and the response here seems to be to sweep them under a rug. How do we get these kinds of things actually fixed (and I use fixed deliberately, as the current behavior is broken to any rational user)?

@Sean Robertson (seanr) I see @Germano Cavalcante (mano-wii) was fast than me. There's a patch in development.

I personally agree that the current implementation is not ideal, at least with the given name. The ticket was originally closed because the very same issue was previously reported in another ticket and closed by a core dev. Additionally, we have very strict rules about improvements suggestions and feature requests, which should usually be posted on right-click select. I admit that I might have been a bit quick in closing this though. I know the "works as design" reply can be very annoying, but we have to draw the line between "improvements suggestion (off-topic)" and "UX so terrible it is considered a bug (on-topic)" somewhere.

@Sean Robertson (seanr) The patch has been committed to the master branch thanks to @Henrik Dick (weasel). Tomorrow's daily build of 2.92 should include this feature, if you want to test it.