I have done thousands of inspections, saw this exact set up for the first time last November. Wrote it up as a major issue, costly repair. Called in a roofer with 25 years of experience and he had never seen it before either but recommended a full removal and installation of a proper deck. Could see blue sky through cracks in the tile. The roofer said he has heard of this type of install before on homes needing a very quick roof replacement after storms. In my case the home was built in 79 and I believe the roof was replaced in the late 80’s or early 90’s after the home had gone through a flood. Tiles were held up with 1x4 decking, spaced about 12 inches apart leaving a 6-8 inch gap in-between each board. Another great reason why I do not walk tile roofs.