Transforming an SO to an Invoice but the tax on the line item is 0 unless I edit then save the newly created invoice record. Anyone got any ideas what might be happening? This might not be relevant but this is on a US account, I've done a similar transform on Australian accounts and it works.

Try creating/transforming in dynamic mode.

what type of script are you using to transform the SO to an invoice? i have run into this before where it was a UE transforming it, and in my case US taxes were calculated by Avalara, which also runs on a UE script - and UEs can't trigger other UEs. so we had to switch to transforming the SO to an invoice in a scheduled script instead of a UE to allow the avalara scripts to run and calculate tax.

@amy what you're saying could well be the issue