Also having this issue with Venu 1 on a Pixel 9a. I had this issue several months ago, it got fixed, and now it's broken again with this latest Connect version (5.19). Watch is connected, then once it disconnects, it never reconnects (until removing the device and re-pairing). Restarting the devices (phone and watch) does not fix for me.
Garmin Connect: 5.19
Venu: SW 7.8, BT 8.02
OS: Android 16 (BPA.251103.015)