Oh. Yeah. It is.
I think it is by mistake.
(It also would not be the first time that Testing was skipped … for kernels, etc)
https://manjaristas.org/branch_compare?q=nano-syntax-highlighting
Under just about no circumstances should Testing be behind Stable.
I raised it over on Update requests for packages imported from Arch Linux - #371 by cscs
EDIT.
@scotty65
According to phil … the package was pushed as a fix to stable but not testing. 