Probably not a bug. From messages on another section we know you've been sending in broker requests. Each time one such request is accepted the reserve value you paid becomes non-refundable and vanishes from your company value.
If you buy the plane(s), then you get a full valued item but pay only the difference from the downpayment. At that time your company value goes back to normal.
That, or someone in accounting is embezzling. Better hire an external auditor.
