If Leap Seconds were added at less frequent intervals since 1999, this implies to me that the rate at which the spin rate of the earth is slowing
due to earth - moon
tidal friction
effects has decreased since 1999, which means there was an opposing
effect that started at 1999
due to a temporary (how temporary?)
So you've gone through and looked at all the individual
tidal gauges, corrected for local
effects like post glacial rebound from the last ice age, local subsidence
due to soil compaction or groundwater removal, local soil buildup
due to
tidal or river silt deposition, discontinuities
due to earthquakes, and the like?