When running dplyr::tally() on a grouped data.frame, then the last grouping variable will be removed from the "Groups". I'm sure that this is intended behaviour but I don't understand why and where this is documented. Any ideas?
library(dplyr) # 1.0.10
# single grouping variable returns ungrouped output
starwars %>%
group_by(species) %>%
tally() %>%
groups()
#> list()
# three grouping variables return output grouped by first and second group
starwars %>%
group_by(species, eye_color, skin_color) %>%
tally() %>%
groups()
#> [[1]]
#> species
#>
#> [[2]]
#> eye_color
As @Konrad Rudolph mentioned tally internally calls summarise whose default behavior is to drop last grouping value.
From ?summarise
.groups [Experimental] Grouping structure of the result.
"drop_last": dropping the last level of grouping. This was the only supported option before version 1.0.0.
"drop": All levels of grouping are dropped.
"keep": Same grouping structure as .data.
"rowwise": Each row is its own group.
Additionally,
When .groups is not specified, it is chosen based on the number of rows of the results:
If all the results have 1 row, you get "drop_last
Since in your case you are getting 1 row per group drop_last behavior is used.