cannot scroll a sheet with 2 fingers on touch pad in Safari

Steps to Reproduce

1. visit keikai with Safari on Mac
2. create a new book
3. scroll a sheet up/down with 2 fingers moving on the touch pad

Current Result

the sheet doesn't scroll

Expected Result

the sheet scrolls with moving fingers

Debug Information

• safari 12.1.1 , Mac 10.14.5
• chrome doesn't have the bug with the same usages on the touch pad.
• using mouse wheel doesn't this problem

Workaround

using mouse wheel