This may not be a issue, but is a different feature from Nvim-R.
devtools::load_all has parameter of export_all, theoretically the functions were exported, but I can't get argument completion unless I specify in NAMESPACE that the function is exported.
For me, since my package may not be shared with others, I will not specifically indicate in NAMESPACE that the function needs to be exported. devtools::load_all makes all these functions exported. This is very convenient.