Tremorsense is described as:
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.
Tremorsense can’t be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs, have this special sense.
Assuming that tremorsense allows a monster to "see" other creatures that are touching the ground as is covered by option 1 of this answer and also assuming that the monster is unable to see the character except via tremorsense, does jumping cause a creature to be unseen by the monster?
As an example Raflannan the thief is facing a fearsome Ankheg in an area covered by heavy fog. Raflannan runs and jumps firing his crossbow at the Ankheg while midair. Is Raflannan’s attack rolled at disadvantage due to not seeing his target or is it a regular roll because the advantage of being unseen cancels out disadvantage?