The narrow region buffer filetype is always detected by Vim, not the same with the original buffer.
Also see this question.
For example, for the following ini file:
key1 = value1
key2 = value2
Select line 2 and 3, type :NarrowRegion, and the new buffer won't be detected as ini file. The syntax highlighting thus won't be corrected done.
The filetype was not always correctly set up. (Reported by Hong Xu in
address startup and lazyredraw related issues (#10)
fix colors not being defined when changing color schemes (#10)
why does osx behave differently from linux T_T (#10)
ensures highlights are applied on window enter (#10)