What is your max spindle speed? I machine literally tons on UHMW. @1750 rpm you are only at ~170 fpm, You want ~500 sfpm and ~ .002"/tooth for a finish pass. What I see in your picture is not uncommon if your going too slow either spindle speed or feed. You can always run a finish pass too to clean it up.
I use accupro, Onsrud carbide ALOT. I run a 1/2" 2 flute 6krpm, .5" doc. 95- 100 "/min.
Hss cutters are frequently sharper edged than carbide and sharp is your friend with UHMW but.. when planning a surface a small radius on the corner of the cutter is a big help.
Roughing feeds of .006 - .01"/ tooth is not uncommon.