First call to transform() fails with inf, all subsequent calls are OK - what could be the reason?

I’m trying to get to the bottom of the following (weird) problem. When applying pyproj.Transformer transformation to a certain polygon, it fails the first time with (inf,inf) throughout. Second and all subsequent calls to the same work just fine!

I saw similar issues with inf which had to do with inappropriate CRS (out of bounds), but this is not the case here.

The question was originally prompted by geopandas’ to_crs() failing occasionally; similar question by someone else here.

Example

import shapely, shapely.geometry, shapely.speedups, shapely.ops, pyproj
print('Shapely version:',shapely.__version__)
print('Shapely speedups enabled:',shapely.speedups.enabled)
#shapely.speedups.disable() # makes no difference
print('Pyproj version:',pyproj.__version__)
print('PROJ version:',pyproj.proj_version_str)

xy = [(532303, 181814), (532190, 181854), (532192, 181861), (532199, 181863), 
      (532195, 181874), (532183, 181870), (532163, 181864), (532162, 181868), 
      (532309, 181915), (532294, 181828), (532306, 181820)] #, (532303, 181814)]
geom = shapely.geometry.Polygon(xy)
OSGB1936 = pyproj.CRS('EPSG:27700')
WGS84 = pyproj.CRS('EPSG:4326')

proj = pyproj.Transformer.from_crs(OSGB1936, WGS84, always_xy=True)
func = proj.transform

# First call fails with inf, all subsequent calls are OK:
for ii in range(3):
    tp = shapely.ops.transform(func, geom)
    print(tp.exterior.coords[0])

Output:

Shapely version: 1.7.1
Shapely speedups enabled: True
Pyproj version: 2.6.1.post1
PROJ version: 7.1.0
(inf, inf)
(-0.0929010345368997, 51.519248856167074)
(-0.0929010345368997, 51.519248856167074)

import os
os.environ['PROJ_NETWORK'] = 'OFF'