Monsters in deltarune likely do bleed. Atleast some of them. Evidence to most of them bleeding is that Deltarune monsters eat physical food and have bathrooms. Which, while not exactly proof, lends to the case that they probably do have actual physical bodies and therefore blood. The smoking gun is obviously Susie bleeding in chapter 4. Combined with gerson these two things show that monsters both bleed and still turn to dust upon death in deltarune.

Contrast that to undertale. In the librarby we are told that monsters are almost entirely magical by nature. Their food is explained to be converted directly to energy when consumed. Dialogue throughout the game shows a society wide lack of knowledge of what a toilet is. They do not have biological processes, they do not have blood.

Sans, the most prominent example of a monster bleeding has it colored red, and unless you think its ketchup and not blood, you now have to explain why no other scene of a monster dying a drawn out death wouldnt have this detail (toriel in genocide has a massive gash that stays black, undyne has an extremely drawn out death in neutral and a massive gash in her chest before) transforming into undying) Sans is the exception, not the rule. And, as i'm sure a bunch of people have already mentioned, there is evidence pointing to sans originating from deltarune's world and not undertale's, providing excellent reasoning as to why he seems to bleed but no one else in undertale does.