ArduCopter 3.6.10 altitude drop while flying with rtk

Flying with rtk, just altitude keep drop and climb, seems like the EKF just not refresh the altitude
log file

For serious situation the altitude Estimate totally stop working making the drone can not land in ALT Hold ,Loiter ,RTL Mode

just found this useful video

@tridge @priseborough @rmackay9 , do you have any idea on this?

just check all the values in dataflash log, only the NKF5.ePos seems to related with EKF altitude none updated

Trying to find out whether the EKF use the gps data or not, is this condition can be replay with dataflash log?