The fourth vowel in rhytadiadelphus
heard as rhytadiadelphus
spelled rhytidiadelphus
Rhytidiadelphus. The syllable after rhyt- is ti, not ta.
What the ear did
It is the fourth vowel of six and carries no stress, so the ear reports nothing more than a blur. The i belongs to rhytid-, the stem of the Greek word for a wrinkle, and it stays i in every word built on that stem.
R-H-Y-T-I-D-I-A-D-E-L-P-H-U-S
The full entry for rhytidiadelphus covers the name itself.