Chrome 56.0.2924.87 (The latest at the moment) only bug (chrome 55.0.2883.75 - no bug)
Testing environment: 64 bit windows 10 64 bit chrome don't know about other envs, 1920*1080 screen
Ext JS 6.2.1.167 - (6.0.2 also has the bug)

Steps:
1) Just open http://examples.sencha.com/extjs/6.2...ffer-grid.html in crome 56
2) Select the first row
3) Scroll down by one mouse wheel portion, wait about 1s - exact time does not matter
4) Repeat (3) untill you see strange jump back by some portion of records

In my case, last visible row takes these values after each scroll: 9 13 17 21 27 29 33 37 41, 19

Video
(download before watching for better resolution)

UP: There are reports about Opera 43 also having the bug