This has been replicated on 3 different TomToms (all up to date) GO 500, GO 520 and GO 520 WiFi.
TomToms are not using the TomTom app on the phone but just using the Bluetooth tethering option. Initially all works good and connection is made with HD Traffic, but at some point in the journey HD Traffic stops working. Turning TomTom off and back on or a full drum roll reboot will not get it working again. The entire journey then takes place with no HD Traffic, defeating the purpose!
The only fix seems to be to reboot the phone and TomTom, where it may start working again, but not always possible to restart my phone if I'm driving on my own.
TomTom reports "Service unavailable" on both HD Traffic and My Drive but is connected to the phone still, with the Pixel XL 2 still showing the Bluetooth tethering icon.
After a lot of time troubleshooting this, the connection breaks if the network drops to 3G (from 4G), and even if going back to a 4G signal, the TomTom will not reconnect to HD Traffic.
1) When connected to a 4G network, have TomTom connect up to Bluetooth sharing, very quickly it will connect to MyDrive and HD Traffic and fetch traffic information.
2) Manually set the phone to use a 3G network (from Network settings on the phone), and to quickly replicate the problem, put the TomTom into sleep then back on again so it drops the current connection and reconnects back to the phone (or leave it for a few minutes and the connections will time out). Whilst the phone shows Bluetooth tethering active, and TomTom shows a connection to the phone, HD Traffic and MyDrive will stick at "Service unavailable" indefinitely.
3) Switch back to back to 4G and notice TomTom still fails to reconnect.
During step 2, where one TomTom has now failed with a constant "Service unavailable", I've started up a second TomTom which has a pairing with the same phone and it works absolutely fine, showing the phone is still passing data and has a network connection on 3G and Bluetooth tethering is working fine. Toggle back to 4G then 3G, and the second TomTom will eventually stop working as well.
Note that this is using the EE network in the UK, it may be specific to EE and their use of IPv6 perhaps. Something about a break in the data connection as the phone switches between 4G and 3G causes the TomTom to get confused and it fails to reconnect again. It may equally be an issue with the Android P and the XL 2 and it's tethering, although I noticed similar issues with Android Oreo and on a Nexus 5X.
I hope this can be raised to your developers to try and replicate the problem and look towards a fix.