Did you change the expression to divide by 7.0?
Are WEEKNUM() and WEEKDAY() returns consistent on both platforms?
I have done a test on both platforms (G Sheet and AppSheet). I used the same expression for the Result column, and this is the result:
CEILING( WEEKNUM([Date]) - ( WEEKDAY([Date] - 1) / 7 + 1 ))
Both platforms deliver a different result for day 7.
Also, when testing the expression, AppSheet shows the right result but delivers a different one (See the image below)
Looks like a bug to me. Please contact firstname.lastname@example.org for help with this.