It's the drains in the window frame (someone will come along and correct me) not leaky glass.
Google tells me with this site as the reference....
VW Transporter (T5/T6/T6.1) sliding windows leak primarily due to a flawed drainage system that fails to handle high water volume, resulting in water overflowing into the van, often due to blocked drain holes or poor bonding between the window cassette and glass. Other causes include dirty or degraded rubber seals and failure to clean debris from the sliding tracks