I am having the same issue.  I just "upgraded" to QuickBooks Plus + QuickBooks Online Payroll Elite.  When I was on Desktop, I was able to separate various reimbursements into the correct accounts; meals, mileage, supplies, etc.  I am disappointed that the field in not editable in this version and instead lumps all reimbursements into one category.  I am even more disappointed to see that this concern was raised 2 years ago and has still not been addressed.