When a command that uses execute to run forceload (e.g. execute run forceload add 0 0) is put inside an .mcfunction file, and the data pack is reloaded, the following error occurs and the function is invalidated and therefore unusable.

java.util.concurrent.CompletionException: java.lang.IllegalArgumentException: Whilst parsing command on line 1: Incorrect argument for command at position 12: ...cute] run <--[HERE]

My personal guess is that execute run doesn't, for some reason, recognize the command, as it is very new.