Code Sample, a copy-pastable example

import geopandas as gpd
from shapely import wkt

# Define 2 geometries
point = wkt.loads('POINT (-3.61104 54.12069)')
polygon = wkt.loads('''
    Polygon ((-3.63156666666667 54.1267166666667,-3.53435 54.0763833333333,-3.5634 54.0383333333333,
    -3.58396666666667 54.0467,-3.60175 54.0521333333333,-3.62465 54.05735,-3.64296666666667
    54.0595333333333,-3.65568333333333 54.0740166666667,-3.6695 54.0932833333333,-3.65568333333333
    54.1054666666667,-3.63156666666667 54.1267166666667))
''')
print('\npoint within polygon?')
print(point.within(polygon))

# Geodataframe with 1 polygon
df = gpd.GeoDataFrame.from_dict({
    'id': [1],
    'geometry': [polygon],
})
# Geodataframe with 1 Point
poi = gpd.GeoDataFrame.from_dict({
    'id': [2],
    'geometry': [point],
})

print('\nSpatial join:')
print(gpd.sjoin(poi, df, predicate="within"))

Problem description

The spatial join behaviour (with GeoPandas PyGeos) has changed between Shapely 1.7.1 (correct behaviour) and Shapely 1.8 (incorrect behaviour).
Note that the point does not fall into the polygon. These are WGS84 coordinates, and the distance from the point to the polygon is ~400 meters.

With Shapely 1.7.1 (correct behaviour)

With Shapely 1.8 (incorrect behaviour)
Note that the point is still considered out of the polygon, but the spatial joins incorrectly considers the point to be in the polygon.

Expected Output

The expected output is the one with Shapely 1.7.1.
I think this comes from the fact that PyGeos & Shapely 1.8 are built on the same GEOS version, hence no conversions apply (the warnings does not appear). Maybe the formats are not exactly compatible.

Indeed, the problem disappears if I force Pygeos to perform the WKB conversion between Shapely & Pygeos geometries.
More precisely, if I set the boolean shapely_compatible to False directly in Pygeos source code, I get the expected behaviour.

More details on the relative issue on GeoPandas: geopandas/geopandas#2198

Note: the problem does not happen with pygeos 0.11.1, that is built on newer version of GEOS, but may happen again in the future if Shapely GEOS version is upgraded.

Output of geopandas.show_versions()

This is the output with shapely 1.8.0. Simply run pip install shapely==1.7.1 to test the working configuration.