I really think this needs looking at sooner rather than later.
I'm justing processing a payroll covering hours worked during Feb, to be paid 14th March.
There are 5 employees on there to be P45'd on 1st Feb. If I P45 them without 'processing' their Feb payroll their leaving date won't show up on the RTI submission which will potentially mess up their tax code going forward. If I 'process' the Feb pay [using £0's] their end date will indeed be included within the RTI submission but they'll end up with tax rebates that they're probably not due. Of course, said rebate should hopefully correct itself in the tax code at some point in the future but I don't think that's acceptable as it leads the employee into a sense of having more money than they actually have.
The only way around it is to switch them all to W1/M1 before processing - far from ideal but it works.
I doubt many others using the system would pick up on the above or the potential solution. I'd say it's a fairly big flaw that really needs sorting sooner rather than later. I can only imagine how many employers think they're P45'd somebody properly when they probably haven't.