Unusual behaviour of dplyr::pull()

[Nikeds]

1. When I use dplyr ::tbl() function to get a table from snowflake and rename all the columns to upper case and then apply pull using dplyr::pull(FSYMTICKEREXCHANGE) it gives me the desired output

The Code : dplyr::tbl(ch, "univ_full_table") %>% rename_all(toupper) %>% dplyr::pull(FSYMTICKEREXCHANGE) (FSYMTICKEREXCHANGE is the column name in my table )

But, When I rename to upper case and then again rename to lowercase and apply pull using dplyr::pull(fsymtickerexchange) it gives the output as null

The Code : dplyr::tbl(ch, "univ_full_table") %>% rename_all(toupper) %>% rename_all(tolower) %>% dplyr::pull(fsymtickerexchange)

Result: NULL