RBF Trees don't always combine, subtree left unconfirmed until expiry

[kiwidream]

Description

In some cases there are RBF trees which do not properly combine to make a 'full tree'. When this happens, one tree is left permanently unmined / unconfirmed and hangs around until the RBF tree expiry comes into effect (24H in default config).

In our testing we had two instances which share an electrs backend and the same bitcoind instance. One instance successfully combines everything, and the other has two distinct RBF trees.

The two instances don't have any difference in config.

Version

a66920c

Steps to reproduce

Not easy to intentionally reproduce, but can be observed in a running mempool instance on mainnet

Expected behaviour

RBF trees should always combine such that at least one TX will be confirmed

Actual behaviour

RBF trees can sometimes be split, and one tree is never mined

Screenshots