We are using a Cisco Meraki VPN - Layer 2 Tunnelling Protocol with IPsec (L2TP/IPsec). My colleague has also tried his iPhone X running iOS 13.2.3 on his Windows 10 surface pro and he has the exact same issue. I have tried 'Reset Network Settings' and 'Reset Location & Privacy' on my phone but to no avail. For clarity, I am not referring to the actual iPhone creating the VPN, I am referring to my Windows 10 Pro laptop creating the VPN when my iPhone is simply being used for personal hotspot.
All other functions of personal hotspot appear to be fine - it's just that the phone is blocking the VPN passthrough. I have tried personal hotspot using USB, Wifi and Bluetooth but no success. I can confirm this by using my wife's iPhone 6 running iOS 11 and personal hotspot allows the VPN connection.
There have been no changes to the work VPN. This was not an issue on iOS 12 or iOS 11. Using my iPhone 8 Plus running iOS 13.2.3, I am unable to connect to my work VPN using the iPhone personal hotspot.