Running it as administrator, there is no text in the filter cases, still disappears after reopening. This is frustrating already, I've never encountered a problem like this on W8.1, as I've had it before.
The problem occurs when you have a corrupted USERDATA.dat in your GTA San Andreas User Files, you will need to replace the file already existing in that directory with the one i'm going to provide you now. I can assure you that the file is 100% harmless, before anyone accuses me of that.
When you install SAMP the folder in your documents is automatically created so I suggest deleting the GTA User Files directory in My Documents then doing a fresh install of SAMP once you have also uninistalled the one you are currently using.