Hi,

>>"I drew some annotative objects(blocks/texts/dimensions/etc) with annotative scaling property of 1:100, 1:150 on file 'A'.

>>"Then I XRefed 'A' into 'B'. I also drew some annotative objects with 1:100, 1:150 on file B."

The annotation scale names are equal, but the definition is different!

Someone changed the predefined standard scales (not a good idea)

>>"There's a scale named '1:100_XREF' and when I set that as the drawing scale it only shows objects in A and not in B."

Sure, you have 2 different things with the same name, so ACAD rename the one -

because you can't have two different things with the same name in one namespace/file.

Check your anno scale definition of 1:100 in your main file and 1:100 in your other file,

or 1:100 and 1:100_xref in the main file.

If I am right, the defintion is differently (and "1:100" or "1:100_xref" is the same like "Tree" "Dog" "Apple" => Just NAMES)