z <- st_sf(pid=5, geom=st_sfc(st_polygon(list(matrix(c(0,0,4,0,4,4,0,4,0,0),ncol=2, byrow=T)))))
x <- st_sf(id=1:2, geom=st_sfc(st_point(1:2), st_point(2:3)), geom2=st_sfc(st_point(1:2), st_point(2:3)))
# OK
st_join(x,z)
Simple feature collection with 2 features and 2 fields
Active geometry column: geom
Geometry type: POINT
Dimension:     XY
Bounding box:  xmin: 1 ymin: 2 xmax: 2 ymax: 3
CRS:           NA
  id pid        geom       geom2
1  1   5 POINT (1 2) POINT (1 2)
2  2   5 POINT (2 3) POINT (2 3)

# No OK
st_join(x[,'geom'], z)
Simple feature collection with 2 features and 1 field
Geometry type: POINT
Dimension:     XY
Bounding box:  xmin: 1 ymin: 2 xmax: 2 ymax: 3
CRS:           NA
  pid    geometry
1   5 POINT (1 2)
2   5 POINT (2 3)