That's what I am trying to say. Your Custom program might not be committing the original LUW created by DELETE_PAYMENT_PROPOSAL.
What I am suggesting is.
1- UNPLUG your code and try deleting payment proposal.
a) if it works
=> you need to manually commit the predecessing LUW. something like "comitt work and wait".
b) if it doesn't work.
=> you might want to raise an OSS or check your config to make sure that your standard process of payment proposal deletion is working.
br,
Kashif