Part of the Route 4 map has no headbutt encounters.

[Stroll 1]

 

Not sure if it's intended but: This map here (the multi-level one) has 5 headbutt trees. Every single headbutt on any tree results in "Nope, nothing..." on every headbutt.
If i change zone to another part of route 4 the headbutts work fine, getting 'mons like aipom, swablu, etc but after approx 100 headbutts on every tree here, several re-zones and random switching, not a single encounter.
Grass works fine. Either i'm missing something or the encounter table is missing for trees here, and i havent heard tell of any special headbutt tree interaction here.