Diagnosis
The bar is remembering a paired Bluetooth device that's now in range and powering on, and it's prioritizing that connection. It thinks you want to switch.
How to Fix It
You need to clear the bar's memory of that Bluetooth device. 1) On the bar or remote, press SOURCE until the side light is solid blue (Bluetooth mode). 2) Hold down the BLUETOOTH button on the remote (or the top of the bar) for at least 5 seconds until you hear a tone. This puts the bar into pairing mode and effectively disconnects any previously paired devices. 3) Now, press SOURCE again to switch back to HDMI ARC (solid red light). The bar should now stay on that input.
If That Didn't Work
Go into the Bluetooth settings on the device that was causing the switch (probably your phone or tablet) and manually forget or unpair the JBL Bar 2.0 from that device's list. This breaks the automatic connection attempt from the other side.
Heads Up
If you want to use Bluetooth again later, you'll have to go through the standard pairing process fresh, which prevents this auto-switching from happening unintentionally in the future.