The INST_ALWAYS_ON flag goes on a different byte than the ISB_* flags, it's the 'Inst' byte just before 'BStyle' and after ClickID. (I'm not sure how you'd get to that with PRISM though.)
struct IS_BTN // BuTtoN - button header - followed by 0 to 240 characters
{
byte Size; // 12 + TEXT_SIZE (a multiple of 4)
byte Type; // ISP_BTN
byte ReqI; // non-zero (returned in IS_BTC and IS_BTT packets)
byte UCID; // connection to display the button (0 = local / 255 = all)
byte ClickID; // button ID (0 to 239)
byte Inst; // some extra flags - see below
byte BStyle; // button style flags - see below
byte TypeIn; // max chars to type in - see below
byte L; // left : 0 - 200
byte T; // top : 0 - 200
byte W; // width : 0 - 200
byte H; // height : 0 - 200
// char Text[TEXT_SIZE]; // 0 to 240 characters of text
};