It’s because Garmin Connect is buggy as ****.
The hardware is good but the software isn’t so good.
I had a drama with my 530 recently – couldn’t get it to connect to the app on my iPhone SE 2020. But my forerunner 645 music had no issues with it.
Ended up having to uninstall connect. Then delete the 530 and 645 from the Bluetooth pairing memory on the phone. Also had to go into the 530 and delete the iPhone from its Bluetooth pairing memory.
Then reinstalled the app and eventually got it to find the edge and pair. Also then had to re-pair the 645.
Bit of a faff but I’m there now.