Welcome to YT.
Yes, it is common in yorkies.
Lactulose is good to help prevent neuro symptoms cause by toxin buildup. There is a lot more to do here though.
Your pup needs to see an internist for diagnosticimaging, other liver support supplements, possible diet change, and to consider surgery if it is a shunt.
In other words, the vetdoes not seem to be taking this seriously enough.