I think that it's not a bug but a feature so I think it's meant to be like this, and basicaly I don't remember but I think this CVar might be cheat protected, and basically all of the r_ (renderer) CVars shouldn't be changed as they are there only left by developers that used them to debug engine in real-time.
There are some valid r_ CVars like r_picmip with values between x - y.
So I think it's not a bug but a natural consequence of using this CVar.
Also what Armageddon says might apply, because all in all, you're giving yourself an advantage that can't be aquired via normal Video Settings Menu.
BTW. Here's a file of CVars that are considered bad (at least were considered by DMW)
Code:
//CHEAT CVARS DEFINITION FILE
//CREATED BY: CHRIS #2 (C)
//
//SYNTAX:
//CVAR_NAME|ALLOWED_VALUE|RANGE|DEFAULT_VALUE
//
//EG: cg_shadows|0|2|1 - means cg_shadows with normal value starting from 0 to 2 (range), default value is 1
//
//When RANGE = . it means that it has no range and only one correct value {old 'n' useless}
r_drawbrushes|1|1|1
r_drawstaticmodelpoly|1|1|1
r_drawstaticmodels|1|1|1
r_farplane|0|0|0
r_farplane_nofog|0|0|0
r_lightmap|0|0|0
r_picmip|0|2|1
cg_3rd_person|0|0|0
cg_shadows|0|2|1
r_drawbrushmodels|1|1|1
r_drawentities|1|1|1
r_drawentitypoly|1|1|1
r_drawspherelights|1|1|1
r_drawterrain|1|1|1
r_drawworld|1|1|1
r_fullbright|0|0|0
r_intensity|1|1|1
r_light_nolight|0|0|0
r_lockpvs|0|0|0
r_mapoverbrightbits|1|1|1
r_nobind|0|0|0
r_nocurves|0|0|0
r_novis|0|0|0
r_singleshader|0|0|0
r_vertexlight|0|0|0
ter_error|0|10|4
r_gldriver|opengl32|opengl32|opengl32
cg_effectdetail|0.2|1|0.8
cg_cameradist|120|120|120
cl_timenudge|0|20|1
cg_cameraverticaldisplacement|-2|-2|-2
r_farplane_nocull|0|0|0
r_ignorehwgamma|0|0|0
r_light_showgrid|0|0|0
r_nocull|0|0|0
r_noportals|0|0|0
r_overbrightbits|0|0|0
r_showbox|0|0|0
r_subdivisions|1|20|4
ter_lock|0|0|0
ter_maxlod|3|10|6
ter_maxtris|16000|30000|24576
r_lodcap|0.25|2|0.5
r_lodviewmodelcap|0.25|2|0.55
r_lodscale|0.25|2|0.55
r_light_sun_line|0|0|0
r_norefresh|0|0|0
r_roundimagesdown|1|1|1
r_showcull|0|0|0
r_shownormals|0|0|0
r_showsmp|0|0|0
r_showstaticbboxes|0|0|0
r_showtris|0|0|0
cl_pitchspeed|35|120|70
cl_yawspeed|35|180|140
r_drawsun|0|0|0
r_fastsky|0|0|0
r_flares|0|0|0
ui_voodoo|0|0|0
cg_acidtrip|0|0|0
3 same values means that it can only be set to that specific value