Part of this was a known issue with LND (your intuition was spot-on nostr:npub186tqtnx8nc502veqkguypvzpm5hq53p69qrl8l0hes9c4k8mzrssfk59su)

If you want to see which inflight payments are actually stuck you can do something like this:

lncli listpayments --include_incomplete |

jq -r '

.payments |

.[]

| select(.status == "IN_FLIGHT") |

[.payment_hash, .value_msat, ([.htlcs | .[] | select(.status == "IN_FLIGHT") | .route.total_time_lock ] | max)] |

@tsv

'

Then check that the max route.total_time_lock is in the future