Question: Why "madulas" and not "marulas"?

Currently, I am not aware of any existing “rule” stating that the first letter of the root word should be changed from “d” to “r” when a vowel comes before it. Some native speakers still use “madunong” instead of “marunong”. Both “madumí” and “marumí” are commonly used. Almost everyone would say “madungis” instead of “marungis”. However, no one ever says “maRilím”, “maRalî”, “maRugô”, “maRayà”, or “maRalang”.

It is possible that in the past, changing “d” to “r” was a common practice such that both “madulas” and “marulas” did mean “slippery” then. (https://wikimapia.org/882257/Marulas). However, other than “Marulas” being used as the name of an area in the city of Valenzuela (formerly a part of Bulacan province) I have not heard anyone use it.

@Juantutri Thanks. So far, "madulas" is the only word I came across where that sound change didn't happen, hence my initial question. Perhaps I overgeneralized from the limited initial-d vocabulary I learned. But now I'm curious: is there a way to tell which words undergo that sound change and which don't, other than rote memorizing din/rin, dito/rito, diyan/riyan, doon/roon, and darating (the ones that I remember off the top of my head), but madulas, madilim, madali, madugo, madaya, and madalang?

@pauamma There is that recommendation that if the preceding word ends with a vowel or with the letters W and Y (because they end sounding like a vowel too) we should use the "r", i.e., raw, rin, riyan, etc., instead the "d". However, that is not strictly followed. I believe that the reason behind that recommendation is only to make the word sequence easier to pronounce. Not following the recommendation does not lead to an error in grammar. So, don't worry about it and I assure you that neither do we. Feel free to use whichever makes it easier for you to pronounce after a word.Just keep in mind that if the words have a prefix, you have to make adjustments depending on which word is used. For example:NANdito = NArito
NANdoon = NAroon
NANdiyan = NAriyan