Why does it keeps disconnecting from the transmitter base?

For Lot Numbers before T22L1:

The HT381 has a power saving function. The transmitter and the headphones will go into standby mode if there is no audio playing for 5 minutes. The headphones will disconnect from the transmitter once they go to standby mode. At this point, you'll need to reconnect them again.

You could check and follow the following steps:

For Lot Numbers T22L1 and after:

Once the headphones and the transmitter are powered on, they will auto pair with each other.

The only time they will disconnect is if the headphones are placed on the transmitter for charging, so you won't encounter the problems mentioned above.