got empty block while txpool is not empty

[liaoyd]

the result is as below:
height: 999 txnum: 0 duration: 1 tps: 0
height: 1000 txnum: 0 duration: 1 tps: 0
height: 1001 txnum: 66 duration: 1 tps: 66
height: 1002 txnum: 306 duration: 1 tps: 306
height: 1003 txnum: 390 duration: 1 tps: 390
height: 1004 txnum: 840 duration: 2 tps: 420
height: 1005 txnum: 1925 duration: 2 tps: 962.5
height: 1006 txnum: 1137 duration: 4 tps: 284.25
height: 1007 txnum: 1075 duration: 1 tps: 1075
height: 1008 txnum: 3000 duration: 2 tps: 1500
height: 1009 txnum: 0 duration: 2 tps: 0
height: 1010 txnum: 0 duration: 1 tps: 0
height: 1011 txnum: 0 duration: 1 tps: 0
height: 1012 txnum: 2238 duration: 1 tps: 2238
height: 1013 txnum: 0 duration: 3 tps: 0
height: 1014 txnum: 0 duration: 1 tps: 0
height: 1015 txnum: 0 duration: 1 tps: 0
height: 1016 txnum: 1023 duration: 1 tps: 1023
height: 1017 txnum: 0 duration: 1 tps: 0
height: 1018 txnum: 0 duration: 2 tps: 0
height: 1019 txnum: 0 duration: 1 tps: 0

between height 1001 to 1016, txpool is not empty, but some block is empty