Hey there tiedbowtie,
Thanks for reaching out to the Apple Support Communities. To get started, you've done some great troubleshooting steps to isolate the issue. Based on the information you've provided, since the Nanoleaf lamps do stop working after moving them outside, it is possible that they're out of range of your HomeKit.
To help isolate the issue further, have you had a chance to check for any updates for your HomePod, iPhone, and Nanoleaf accessories? If not, check out the steps listed in the following resource that can help with updating your devices:
Update your iPhone or iPad - Apple Support
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. If you don't know your passcode, learn what to do.
Update HomePod - Apple Support
1. Make sure that your device is updated to the latest version of iOS, iPadOS, or macOS.
2. Open the Home app.
3. Tap or click the More button
, then Home Settings
> Software Update.
4. If there's a new update, select Update. If there isn't a new update, you can select HomePod to turn on automatic updates and get the latest software automatically when it's available. Software Update also tells you if your HomePod is already up to date, and shows you the installed HomePod software.
For help with checking for updates for your Nanoleaf accessories, you may need to download the manufacturer app, or reach out to the manufacturer for steps on how to update.
Another step we would recommend would be to restart any modem, router, and Wi-Fi signal extenders that you're using, as this can refresh the connection with your accessories.
Keep us updated on the results after going through the steps above, and we'll continue from there.
Have a great day!