The STR_SPLIT function produces additional columns when the treated string contains sequential paragraph ('§') characters.
For instance, using the following script:
:DEFINE &STRINGS#, string, 5
:SET &STRING# = "123_456§§§§789"
:FILL &STRINGS#[] = STR_SPLIT(&STRING#,"_")
The expected result might be:
123
456§§§§789
but the actual result is:
123
456
§789
where the string is split on the underscore ('_') but also on '§§§', the fourth '§' is then considered a valid character again.
The AE processes internally use three sequential paragraph characters (§§§) as column separators.
This is (indirectly) documented here:
A hack is shown in the example here:
This is by design and '§§§' cannot be used or should be temporarily replaced.
The same may happen in other UC4 script functions.