DIAGNOSIS This is a signal interference or power-saving issue. Bluetooth is a short-range, low-power signal that can be disrupted by other wireless devices, or the phone might be putting its Bluetooth to sleep.
FIX Minimize interference. Keep the label maker and your phone close together (on the same desk, no obstructions). Move them away from routers, microwaves, cordless phones, or large metal objects. On your phone, go to Bluetooth settings, find the D30 device, and see if there's a setting like "Keep Alive" or disable battery optimization for the Phomemo app (in your phone's battery settings) to prevent it from being put to sleep.
IF THAT DIDN'T WORK The battery in the D30 might be dipping to a low level where it prioritizes power. Charge it fully. Also, try forgetting the device on your phone and re-pairing it as described in the connection question above—a fresh pairing can sometimes stabilize a flaky connection.
HEADS UP If you're in a busy environment like an office with dozens of Wi-Fi and Bluetooth devices, dropouts are more common. For critical printing jobs, consider designing and saving the label in the app first, then turning the D30 on, connecting, and printing immediately to minimize connection time.