tmap is only plotting the first polygon in an sf geometry column, but plot() gets them all.
Some github issues for tmap and leaflet mention something similar, but they were related to names in an sf geometry column (which the World data doesn't have). Additionally, the problem was fixed in both packages.
Here's a reprex using the first example in the tmap Getting Started vignette.
library(tmap)
#> Warning: package 'tmap' was built under R version 3.5.3
data("World")
# tmap only plots one polygon
tm_shape(World) +
tm_polygons("HPI")
# plot gets them all
plot(World["HPI"])
Created on 2020-10-15 by the reprex package (v0.3.0)
devtools::session_info()
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#> ctype English_United States.1252
#> tz America/Chicago
#> date 2020-10-15
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I was with the same problem. So, I upgraded to the recent version of the tmap package and R version. The problem was fixed.
R version 4.1.0 (2021-05-18) Tmap package version 3.3-1