BYOND has a large number of built in flags, and I assume that the fairly new BYOND visual filter flags have yet to get added to a list of built in flags. The result is that using one of those flags results in an erroneous unknown variable error, when it compiles just fine. The flag in question is OUTLINE_SHARP for the outline filter, as seen in this picture.

After doing some research, the built in flag definitions seem to be located here so I'm not sure if it's as simple as just adding them so the language server stops thinking it's an undefined variable. If it actually is just that simple then I can try to fix it, and #195 seems to suggest that it would be an easy fix, hopefully.