DIAGNOSIS A software bug in the remote's firmware causes it to lose sync with the Chromecast device.
FIX Unpair and re-pair the remote. Go to Settings > Remotes & Accessories > Remote. Select 'Pair remote' and follow the on-screen prompts. This forces a fresh Bluetooth connection.
IF THAT DIDN'T WORK The remote's firmware may need a manual update. With the remote paired, go to Settings > Remotes & Accessories > Remote > Check remote update. Install any available update.
HEADS UP Do not use rechargeable batteries if they are not 1.5V. Many rechargeables are only 1.2V and cause low-power performance issues. Use fresh alkaline batteries.