Well, I will be damned, I decided to check what CSS(3) spec had to say about this, and it says:
The computed values of ‘overflow-x’ and ‘overflow-y’ are the same as
their specified values, except that some combinations with ‘visible’
are not possible: if one is specified as ‘visible’ and the other is
‘scroll’ or ‘auto’, then ‘visible’ is set to ‘auto’.
So, in short, yes, what I was experiencing was entirely the expected behavior.
Source: CSS basic box model W3C Working Draft 9 August 2007 (just after the example)
