Upgrading version should lead to bug - right?

I have an XSLT 1.0 stylesheet that has been running on an XSLT 2.0
processor no problem but barfs when you upgrade the version attribute
(no other change to the code) to 2.0.

That shouldn't happen right?

I've not pinned down the precise line of offending code but the error
I am getting is

A sequence of more than one item is not allowed as the first argument
of contains() ("_IOS, _WEB, _CTV", "_IOS, _WEB, _CTV", ...)

Now the input XML has elements like this

<m__category>_IOS, _WEB, _CTV</m__category>

so I wonder if the processor is parsing that element as a sequence
instead of as a comma separated string.