It looks like a new bug has been found in Fortnite, where sprinting is cancelled when players quickly cycle through their weapons, whilst jumping. Epic are aware of this bug as they have update the Official Fortnite Trello Board. At the moment, it is still being investigated, but we will let you know where this bug has been fixed. Here is what the Trello Board shows:
This issue can also happen when a player stops any movement input (even while midair) and consistently spams inventory swapping. Upon landing, they will be unable to sprint until the inventory swap spam is done.