SBG
I've just got my June, yes that's right, June payslip and noticed this GPD Taxable debt in the bottom right hand corner. The amount was £733, so being a bit concerned I called the JPAC helpdesk and asked what it was. They gave me the same explanation that you have posted, i.e. loads of people in a similar position and you will get repaid end of August. Not good enough I said, there's 10 days to sort it out - we'll get back to you!! Not holding my breath, but I will be registering a formal complaint - and anybody outh there should do the same!
SDF