HMMMmmm…
When (how) did it get there?
https://en.wikipedia.org/wiki/Ark_of_the_Covenant#Possible_locations
The Mercy Seat fabricated under Moses' supervision was only a copy, a simulacrum, an earthly representation of that Permanent one in Heaven which pre-existed the one on earth. Wherever the earthly one is now is of no consequence, for that old system is quite fully wiped out and replaced by the Second, New Will/Testament/Covenant of God, with the True Mercy Seat firmly located in Heaven, as it always was, is, and always will be.
Hebrews 8:4-6 (AV):
4 For if heJesus were on earth, he should not be a priest, seeing that there are priests that offer gifts according to the law:
5 Who serve unto the example and shadow of heavenly things, as Moses was admonished of God
when he was about to make the tabernacle: for,
See, saith he, that thou make all things according to the pattern shewed to thee in the mount.6 But now hath heJesus obtained a more excellent ministry, by how much also he is
The True Mercy Seat of Heaven didn't "get there." It was always there, of which the earthly one was only a temporary copy of, for God's purposes with mankind in this realm.