John Mueller talks about Mobile & Desktop parity when it comes to title tags :

We only use the content visible to mobile Googlebot, so if you have different titles, we’d use the mobile ones.

— 🍌 John 🍌 (@JohnMu) November 7, 2018

Transcript:

“We only use the content visible to mobile Googlebot, so if you have different titles, we’d use the mobile ones.”