Welcome! It's great to see you around @JeroenZ.
Thanks for the details shared on your post. If your iPhone 13 Mini is running the latest iOS version, but it is not able to find your Charge 5, I'd recommend checking the following:
Make sure the Fitbit app is up-to-date. For more information, see How do I use the Fitbit app?
If you have multiple Bluetooth products like speakers or headsets paired to your phone along with your Fitbit device, you may have trouble using more than one at the same time. Turn off other Bluetooth connections when not in use.
If you use more than one phone or tablet to sync, make sure the other device isn't nearby. If the device is nearby, turn off Bluetooth on it. Since the Charge 5 is properly syncing with your iPhone 8, let's try to turn off Bluetooth and then try to sync with your iPhone 13 mini.
On a side note, you can sync your device with any compatible phone, tablet, or computer that meets syncing requirements. If you receive notifications from your phone on your Fitbit device, you'll need to disable the Bluetooth bond between your phone and Fitbit device before using a different phone. For instructions see Why won't my Fitbit device sync?
Hope you find this information helpful.
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