Strange file output when using Concat in CodeSys

I'm using the CAA-File library in CodeSyS to write data to a text file. When I write a string to the file it works fine, but if I combine 2 strings with CONCAT, I get a strange file output with random characters (see below). Does anyone know how to fix this? Thank you!

For example lString = 'test' works fine, but lString = CONCAT('str1', 'str2') does not.

PROGRAM initDataLogger
    VAR_INPUT
        lString : STRING;
    END_VAR
    VAR
        lString : STRING:= CONCAT('str1', 'str2');
        xFileStdInit:   BOOL:=FALSE;
        uiFileStdState: UINT:=0;
        sFileName:  CAA.FILENAME:= '/var/www/html/TestFile.txt';
        hFile:      CAA.HANDLE;
        szFileSize1:    CAA.SIZE := 0;
        szFileSize2:    CAA.SIZE := 0;
        filop:      FILE.Open;
        filwr:      FILE.Write;
        filcl:      FILE.Close;
        loop: INT := 0;
        fString: STRING;
    END_VAR

IF NOT xFileStdInit THEN
    filop(xExecute:=FALSE);
    filcl(xExecute:=FALSE);
    filwr(xExecute:=FALSE);
    xFileStdInit:=TRUE;
    uiFileStdState:=0;
ELSE
    CASE uiFileStdState OF
    0: (* create a new file *)
        filop.sFileName:=sFileName;
        filop.eFileMode:=FILE.MODE.MAPPD;
        filop.xExclusive:=TRUE;
        filop( xExecute:=TRUE);
        IF filop.xDone THEN
            hFile:=filop.hFile;
            uiFileStdState:=1;
        END_IF
        IF filop.xError THEN
            (* error handling*)
            ;
        END_IF
    1:(* write text in the file *)
        //fString := CONCAT('$R$L', lString);
        fString := lString;
        filwr.hFile:=hFile;
        filwr.pBuffer:=ADR(fString);
        szFileSize1:=SIZEOF(fString);
        filwr.szSize:=szFileSize1;
        filwr.udiTimeOut:=100000;       (* 100ms Timeout *)
        IF GVL.logData THEN
            filwr( xExecute:=TRUE);
        END_IF
        IF filwr.xDone THEN
            uiFileStdState:=2;
            GVL.logData := FALSE;
        END_IF
        IF filwr.xError THEN
            (* error handling*)
            ;
        END_IF
    2:  (* close file  - TestFile.txt *)
        filcl.hFile:=hFile;
        filcl( xExecute:=TRUE);
        IF filcl.xDone THEN
            uiFileStdState:=3;
        END_IF
        IF filcl.xError THEN
            (* error handling*)
            ;
        END_IF
    3:  (* end of example *)
        xFileStdInit := FALSE;
    END_CASE
END_IF

For lString = CONCAT('str1', 'str2') I expect str1str2 as output but I get this:

str1str2sys??v$??v??v?
?vd?n??
?v?`v??vd
?vK
?v?`v8
?v?v4??v?

I found it out myself. It had to do with the string size. For example: STRING(4) outputs fine with a four letter word, but with a two letter word it adds two random character's.