Certain forms of caligraphy get closer to poetry (like 漢詩), so it could use phrases and sequences of Kanjis that you won't see anywhere else. As caligraphy is a form of art, it is more interesting to intentionally deviate a bit from common words, because it gets the reader thinking.
I'd consider 一味一禪 to be one of those. I can only guess its meaning, but my guess would be something like "every tea making is an act of zen", or something close to 禅茶一味. Its use of 異体字 (different more traditional forms of the same kanji) 禪 not 禅 also adds another angle to the deviation.
Whether one calls it "correct" Japanese or not is red herring, in my opinion. But you probably don't want to use it in e-mail or other everyday writing.