debugonce() functions are not considered as debugged

debugonce() sets a different flag (RSTEP), and this is not queried by
isdebugged(), and it is also not unset by undebug().

Is this expected? If yes, is there a way to query and unset the RSTEP flag
from R code?

? f <- function() { }
? debugonce(f)
? isdebugged(f)
[1] FALSE

? undebug(f)
Warning message:
In undebug(f) : argument is not being debugged

? f()
debugging in: f()
debug at #1: {
}
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