I think you are using different projections. It looks like the CSV file is in WGS 84
and and the grid is in WGS 84 / Pseudo Mercator
projection. In order to have the points in polygonPoints in polygon
tool works well you need to unify the projections into one projection. In other words, you need to change the projection of one of them into the other.
If you want to use WGS 84, then you need to right-click the grid shapefile -> Save as -> CRS
and select the WGS 84
: