Aha, I think I've found the problem. Quite simple too:
rem animation UVs
tframe=decalelement(f).frame
ty=tframe/decal(decalid).across : tx=tframe-(ty*decal(decalid).across)
q#=1.0/(decal(decalid).across+0.0) : tx#=tx*q# : ty#=ty*q#
rem X10 use fU and fV [ and fSize ] to handle UV control from constant, rather than vertexdata write!
` set effect constant float 12,"fU",frameu
` set effect constant float 12,"fV",framev
lock vertexdata for limb tobj,0,1
rem x10 new plane
if 1
set vertexdata uv 0,tx#,ty#
set vertexdata uv 1,tx#+q#,ty#
set vertexdata uv 2,tx#,ty#+q#
set vertexdata uv 3,tx#+q#,ty#
set vertexdata uv 4,tx#+q#,ty#+q#
set vertexdata uv 5,tx#,ty#+q#
else
set vertexdata uv 0,tx#+q#,ty#
set vertexdata uv 1,tx#,ty#
set vertexdata uv 2,tx#+q#,ty#+q#
set vertexdata uv 3,tx#,ty#
set vertexdata uv 4,tx#,ty#+q#
set vertexdata uv 5,tx#+q#,ty#+q#
endif
unlock vertexdata
endif
endif
so that fix will be in the next beta.
Who says famine has to be depressing?