Fulfillment workbench over allocating stock to sales order lines

hi Folks

we are getting random issues where stock is getting over shipped on sales lines that are fulfilled over multiple deliveries.
example i have is a sales line for qty 240
i have an initial 4 packs shipped with quantities totaling 164 - all shipped and invoiced over a time span of 4 weeks, leaving a back order quantity of 76

then the last pick that get released is for 120! which is then picked and shipped to the customer, and only picked up via an invoice query from the customer

we test this in isolation, and we get no opportunity to over allocate - epicor warns & Stops us if we manually increase the quantity beyond the remaining balance

has anyone else experienced this? it seems random, but we are getting a few examples reported, and who knows how many are going un noticed!

thanks

mal