Hi,
The category parameter is just not set when the information about the order is sent to Google Analytics.
It's not related to variants. Products without categories also don't send the category information.
The GA documentation didn't say how should that parameter be handled. Is there any restriction ? Can we add anything in that parameter ? And mentioned that that parameter was optional. So when we first developed the plugin, we decided not to implement it.
And so far, no one had a problem with that.
Is that a problem for you that the category is not reported to GA ?