It's well known that past a certain point, Latin "t" developed an assibilated pronunciation when followed by "i" and then a vowel, as in the word grātia. Sources agree that there are some exceptions, most of which make intuitive sense to me; e.g. -tīV- is supposed to have resisted this development, and in -stǐV- and -xtǐV- the -t- is supposed to have remained non-assibilated.
But one thing that puzzles me is that various sources say that -ttiV- was immune to this development. From a theoretical standpoint, I can't see why a preceding -t- would be expected to prevent assibilation. The sequence -ttiV- seems to be pretty rare, so I'm also not sure about whether we actually have much evidence to support this generalization about the natural development of the pronunciation of -ttiV-. (E.g. is there compelling inscriptional or Romance-based evidence?)
Walter Blair's Latin Prounciation (1873) says "For manifest natural reasons it is commonly agreed as improbable that the T could have been hissed when it was immediately preceded by another T, or by a hiss, as S or X. (See Zumpt's Lat. Gr., p. 7" (p. 119).
I confess that I haven't sought out Zumpt's grammar yet, as I thought that the members of this site might be able to tell me more quickly and in more detail whether there are actually solid reasons for supposing that -ttiV- was a special environment where the t > [ts] sound law did not apply. If anyone wants to do more digging and post an answer summarizing the results of their research, I would appreciate that as well.