I thought that too, but I just asked a Tanzanian Swahili teacher who replied:
"There are a few borrowed adjectives such as tajiri, hodari, masikini etc. which do not require prefixes. For the adjective ‘safi’, it requires the agreement prefix for many nouns (I know there some exceptions where it does not, e.g. maji safi ‘clean water’). Other examples showing agreement between subjects and adjectives are:
Kiti ni kisafi / viti ni visafi
Tunda ni safi / Matunda ni masafi"
I was really surprised, as every text book I can find says the adjective 'safi' does not take an agreement prefix. However, I then found lots of examples of "msafi" here: