Greetings James Yates,
Updating your iOS can often help to resolve unexpected behaviors, such as what you’ve described occurring with voice navigation in Maps. To start, create a current backup to ensure your data remains secure: How to back up your iPhone, iPad, and iPod touch
Then, use the following steps to update your iOS:
"Update your iPhone or iPad wirelessly
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5.Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now." Update your iPhone or iPad
If you are having issues while connected to Bluetooth, test to see if the issue continues when you disconnect from Bluetooth. This will help you to isolate the issue and determine your next steps.
Should the issue continue, please Contact Apple Support for help with using Siri to show the Weather. They will be in the best position to support you.
Sincerely.