Contr_Conn is right, you are not working with the same data types between the AOI and the rung below it. In the AOI you are working with just SINT (one character of a string). In the rung below that you are working with the entire string even though you are addressing it to the element level.
See if this helps. Change the SourceString in your AOI to V_Read_RT.DATA[8] and see what V_AnodeCurrent_Test reads. I would suspect your output to be 7.0. (Please actually try this and report back what actually happened, I am curious if I was correct)
In the rung with just the AOI you are sending in V_Read_RT.DATA[1] and then copying that value into String.DATA[0], which again is just one element of the string. The tag String has no idea what the other elements of V_Read_RT because it was only told about the 0 in element [1]. The CPS instruction copies a 0 into String.DATA[0], the length of String is changed to 5, and then the STOR instruction should be something like Source: String = '00000' Dest: DestReal = 0.0 because 00000 as a real is still only 0.0.
In the rung below that, you are copying the contents of V_Read_RT.DATA[7], [8], [9], [10], and [11] into tstString.Data[0], [1], [2], [3], and [4]. tstString is '07.89' which the STOR instruction converts to 7.89.
So, you either need to change your input to be a STRING type rather than SINT, or just use the MID instruction to extract the information you need as Contr_Conn already suggested. The MID instruction is pretty easy to use, check out the instruction help for it. I have attached an example of how I duplicated what I think is your end goal for you at the bottom. If you still want this to be an AOI, you could create inputs to dictate the start position of the string, and the length you want to extract before converting to a real, but this seems so much easier than an AOI especially if you are going to be pulling from different parts of the string.