Usually barcode readers can operate in either HID mode (for compatibility with all Apps) or via dedicated channel (best user experience).
In HID mode they're emulating a keyboard, which is probably interacting badly with Compose in this case. Likely the configuration is emitting tabs, newlines or other control codes along with the barcode digits themselves, and that's causing a focus change. See if you can initialise the reader in a different mode.
On handheld wireless scanners this usually means connecting Bluetooth in RFCOMM Bluetooth profile (emulating a Serial port), but for an internal Zebra scanner I'd expect some SDK function.
Overall I agree with
@François this is almost certainly a Zebra Scanner issue and not a Kotlin one.
See:
Zebra Scanner SDK for Android