I believe that is only true if the attribute in the source note doesn't already exist in the target note (I've just re-tested this in 6.5.0). In v5 you could drag attributes between the attributes palettes of different TBX but now, in v6, with a single Inspector that is no longer possible. Thus, you need to ensure all attribute (to be copied are added to the target TBX before you paste in items from the source TBX. Sadly, there is no mechanism for copying one/several attribute with all settings from one TBX to another (I guess that's a feature request at this point).
I presume the behaviour of
intrinsic attribute values may be variable, e.g. things like $Xpos, $Ypos, $Height and $Width may be not all work the same.
Another potential confusing factor is if you have a custom attribute as a Key Attribute in the source TBX and past the note to another TBX (without the attribute), the 'missing' attribute still appears in the KA table but note is it in grey - actually indicating it is missing. This is because $KeyAttributes is a Set of text strings of attribute names. When rendering that list as a KA table, it assumes the user knows best and includes those items because that's what the user asked for.
I think that's a truer answer and I need to update the aTbRef page re user attributes. It was there but in another note that I think got deleted when updating the resource for v6.