stringi binary for 1.2.4 is not available, only the source. As you are on windows, it will install by default the binary, so the previous version 1.1.7.
So currently, it is possible that you install 1.1.7 each time instead of 1.2.4.
Have you the tools to build from source on windows (Rtools) ? If so, you can answer yes to built from source when it ask or run install.packages("stringi", type = "source") in a clean session. Only uncertainty: not sure if stringi compile nicely on windows or not.
However, don't worry if you don't have the latest version of stringi. Everything will go fine unless you really the very last change. When it will be available as binary, you will be able to update it.
And if you really need last version, you can follow the workaround mentioned in github issue