Why is 灸 pronounced jiu1? The pinyin of 灸 as in 针灸 (acupuncture) or 艾灸 (moxibustion) is clearly given as jiu3, as shown in, among others, MDGB and Zdic.
The single character doesn't even have alternate pronunciations, only jiu3 is listed.
However in speech I've always heard it pronounced jiu1 (example: https://youtu.be/RJFsJi7JvFI?t=10). In this other video though (https://www.youtube.com/watch?v=9lJhs4vz4eI) it's pronounced as the dictionary suggests.
Is jiu1 a pronunciation of southern dialects? How did it come about?
This is not an answer but some references that might be helpful and are too long for the comments section.

This is clearly a phenomenon because the first note written on 灸 in《规范》says:

注意 ㊀不读jiū。

But, without much of an explanation it is hard to see why they mention it other than the obvious idea that it often gets mispronounced this way.

As for southern dialects《现代汉语方言音库 • 成都话音档》has the following entry:

灸
tɕiəu⁵⁵

The tone is noted as:

⁵⁵

Which is clearly 阴平, the same as jiu1.

Looking through the tones on: 灸 | Wiktionary we also have:
A high flat tone:

Hakka = /ki̯u⁵⁵/

and a middle flat tone:

Cantonese = /kɐu̯³³/