When setup with a progress type billing rule, we noticed that when the invoice proposal is created it correctly assigns the billing rule.

But if you create a fee journal manually using the same category and bringing it in, billing rule is not associated correctly. Same happens if you try to use the 'Create Fees' on a current open invoice proposal.

Billing rules populating correctly on these transactions is essential because the Progress billing contract is tracking the contract & invoiced values against billing rule ids'. So if there is a data mismatch, the expected fee in the next round of billing is incorrectly calculated.