This happened to me a while back too. Was also disappointed that I lost the size of the channel.

These days if I want to pay an on chain address I use a swap service like Boltz and send via LN. If I want to increase the size of the channel just send onchain sats to the onchain address in Phoenix which will splice it into the channel.

lightning

Understanding Phoenix Wallet liquidity management

martinbarilik

This happened to me a while back too. Was also disappointed that I lost the size of the channel. 

These days if I want to pay an on chain address I use a swap service like Boltz and send via LN. If I want to increase the size of the channel just send onchain sats to the onchain address in Phoenix which will splice it into the channel.