According to dictionaries, the correct pronounciation of "馄饨" is hún tún, but I have never heard any native speakers pronounce the word this way; instead, almost all native speaks say hún dùn. Why?
Because native speakers don't actually know the tone without being taught it.
Northern Mandarin pronounces it as
húndun, with the second syllable having the light tone (轻声). The morpheme 饨 is not used anywhere else, so it only appears in native speech without a tone.
It's not rare for native speakers to reinterpret this syllable as "originally having the 4th tone" because of the acoustic similarity when pronouncing it with the light tone (there's a slight phonetic fall particularly as it follows a 2nd tone syllable). So when they try to shake off the light tone accent which is associated with being regional vernacular, they arrive at
d is unusual. It may be a sporadic change. The only other word I can think of, where the initial (声母) may be lenited in a light tone, is 糊涂
húdu. Many people do "correct" the pronunciation incorrectly to
húdù, but this problem here is less severe because, unlike 饨, 涂 is widely used elsewhere with the known pronunciation of
tú. Hence when speakers (especially literate ones) try to enunciate it, they arrive at
馄 (hún) in '馄饨'(hún tún) is pronounced correctly
Since 饨 (tún) looked like 沌 (dùn), people may have just confused the two characters.
Cantonese write 餛飩 as 雲吞 with the same pronunciation
雲吞 /wan4 tan1/ (Cantonese) / hun2 tun1/ (Mandarin)
餛飩 /wan4 tan1/ (Cantonese) / hun2 tun1/ (Mandarin)
Let's check comprehensive source like zdic.net for 馄饨.
You will notice hún dun ㄏㄨㄣˊ ˙ㄉㄨㄣ come with 国语辞典 tag. This mean the pronunciation is generally used in Taiwan.
While hún tún is used by mandarin speaker in China.
In fact, you can call this "mandarin dialect", though some Taiwanese are not happy with it if you say so.
Just stick to what you have observed, this is a long story, many pronunctions in the dictionary have this problem.
You can observe these characters in your life:
This is a matter of habit. Many Chinese pronounce it wrong，But people are used to it and don't affect communication.