This happened to me twice - found a boat in the ocean (rowboats) both with existing sails and lights. When moving to the new boat and trying to unequip the sail and light from old boat, both sails disappear.
Steps to Reproduce
Try to unequip the sail of one boat while on another also with a sail
Try to unequip the sail of one boat while on another also with a sail
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.