This is an effect of the channel reserve.
The channel reserve is enforced to ensure each counterparty always has something to lose, if they were to try to cheat. By default, the channel reserve is 1% of the channel capacity, and LDK sets a hard minimum of 1000sats.
Therefore, if you were to send enough such that more than max(1% of channel capacity, 1000sats)
were on your counterparty’s side, your available balance for receive would increase.
Since you only sent 1500msats (1.5sats), the amount is still below the channel reserve for the counterparty and none of their funds can move to you.
ldk-sample
, can you show the output oflistchannels
commands?