Post Sourcing trigger fires before custom field is sourced

Hello,

We have a custom transaction column field that sources itself from the item record. On the Transaction form I have some javascript attached with a Post Sourcing trigger that populates other custom transaction column fields.

The problem I have is that the post sourcing javascript appears to run before the custom field has been sourced.

I was wondering if anybody had experienced anything similar to this. I have spoken to support but they tell me that they can't re-create it and I've not heard from them since. However, I have re-created it in our (relatively) clean test account.

Getting an answer form support is more frustrating because the person who my case escalated to works US hours. I don't work US hours (I work in the UK!).