This is the documented behavior. From the ?as.Date:
Format: The character string. If not specified, then it will try '"%Y-%m-%d"' then '"%Y/%m/%d"' on the very first non-'NA' element, and then it will give an error if neither works.
as.Date("09 Jan 2019") gives the error as the format is not in the one of the two listed above. as.Date("01/01/2000") gives an incorrect answer as the date is not in the one of the two formats listed above.
I always take "standard unambiguous" to mean "ISO-8601". If you are receiving above error, then the solution for that is to specify the format of your date in, using the correct formats described in the ?strptime.
Hope my solution helps in resolving your issue.