TTFStems direction reset on reopening

It seems the TTFStems directions are being reset to Y direction upon reopening a file. I can confirm they are being save correctly:

name = TTFStems;
value = (
{
horizontal = 1;
name = "Y: lowercase o";
width = 45;
},
{
horizontal = 1;
name = "Y: uppercase O";
width = 49;
},
{
horizontal = 0;
name = "X: lowercase n";
width = 78;
},
{
horizontal = 0;
name = "X: uppercase H";
width = 83;
},
{
horizontal = 0;
name = "X: uppercase O";
width = 92;
},
{
delta = {
"{100, 74, 0, 0, 0, 0}" = {
"24" = 8;
};
};
horizontal = 1;
name = "Y: lowercase n";
width = 66;
}
);

However, when I reopen the file and save it again, all stems revert to horizontal = 1:

name = TTFStems;
value = (
{
horizontal = 1;
name = "Y: lowercase o";
width = 45;
},
{
horizontal = 1;
name = "Y: uppercase O";
width = 49;
},
{
horizontal = 1;
name = "X: lowercase n";
width = 78;
},
{
horizontal = 1;
name = "X: uppercase H";
width = 83;
},
{
horizontal = 1;
name = "X: uppercase O";
width = 92;
},
{
delta = {
"{100, 74, 0, 0, 0, 0}" = {
"24" = 8;
};
};
horizontal = 1;
name = "Y: lowercase n";
width = 66;
}
);

Thanks, I fixed it.