When using the built-in dictionary, Synthesizer V usually alternates automatically the pronunciation of “the” between /dh ax/ or /dh iy/, based on whether the next word starts with a consonant or a vowel phoneme, respectively.
As an example, “the surface” is converted automatically into phonemes as /dh ax/ /s er f ax s/.
However, I created a custom dictionary and registered “surface” as /s er f ih s/, and now “the” is being converted as /dh iy/ instead of /dh ax/, even though the word still starts with a consonant phoneme, and I didn’t create any entry for “the” in my custom dictionary.
This behavior seems to happen when “the” is followed by any entry in a custom dictionary, as well as notes that use the special “.” lyrics to input phonemes directly.