The Rambam in Sefer Hamitzvot לא תעשה number 198 and #199 counts a prohibition of eating chametz and a seperate prohibition of eating chametz that has been absorbed in a mixture. The Ramban asks why is there a separate mitzvah not to eat חמץ בתערובת, if there is an amount of כזית בכדי אכילת פרס then by all issurim it is considered as if the issue is intact, we should not need a special prohibition here and if it's less than that amount, it is nullified?
The answer can be found in the laws of chametz. It is subject to debate how to understand the rulings of the Rambam. But it is possible that the Rambam rules according to the opinion of R' Eliezer in the Gemarah Pesachim (42-43) that chametz which is in a mixture carries the lav but not the punishment of kares. This is also fits with the Michelta that the Rambam himself in Sefer Hamitzvot 198 quotes ולשון המכילתא: "כל מחמצת לא תאכלו - לרבות כותח הבבלי ושכר המדי וחמץ האדומי. יכול יהו חייבין עליהן? תלמוד לומר: "חמץ" - מה חמץ מיוחד שהוא מין גמור, יצאו אלו שאינן מין גמור. למה באו? לעבור עליהן בלא תעשה. It clearly states there is a lav but no kares for chametz in a mixture. Once we see that there is a מיעוט to exclude the mixture of chametz from kares we would have said that chametz in a mixture does not have a שם חמץ. Therefore, we need a new derasha to renter the chametz into the status of issur even in a case of כזית בכדי אכילת פרס. However, it still needs explanation for in a תערובת there is a halacha of טעם כעיקר telling us that the taam of chametz is considered like the chametz itself so how can we say the derasha is referring to and excluding a case of כזית בכדא"פ?
Why would there be this split that there would be a lav but no kares? We see that there are two aspects to the issur of chametz. Chametz is prohibited like any other forbidden food and is there is a כזית בכדי אכילת פרס it is assur. However, the kares punishment is meted out only for eating pure chametz. The kares punishment is not for eating merely forbidden chametz but for eating pure chametz. In other words, for the lav one is obligated on the mere taste of the issur, it doesn't have to be a cheftzah of chametz but for the kares one is obligated only for eating a cheftzah of chametz. In light of this the Rambam understands that this split to establish the lav separate from the kares is derived from כל מחמצת. That introduces a new issur on the חמץ בתערובת even though it is not bonified kares prohibited chametz.
The Minchas Chinuch (12) points out that the Rambam in the list of the mitzvot in the beginning of Chametz U'Matzah mentions the issur on chametz from erev Pesach but not the issur on a mitzure of chametz. He derives from there the issur of כל מחמצת only kicks in on Pesach itself. This would fit well assuming the issur on תערובת חמץ is its own unique issur and not just a גילוי that it is the regular issur of chametz.
The Rambam (1:7) says האוכל מן החמץ עצמו בפסח כל שהוא הרי זה אסור מן התורה שנאמר לא יאכל. ואף על פי כן אינו חייב כרת או קרבן אלא על כשיעור שהוא כזית. The Achronim aks why do we need a possuk to tell us that the issur of chametz is even on a minute amount, that is the general law of חצי שיעור אסור מן התורה? According to this, the Rambam may have understood that chametz that does not meet the bar of a chiuv kares because it is less than the shiur doesn't have a qualitative amount to be defined as a cheftzah of chametz and therefore a new possuk is required to determine it is assur.
The prohibition on a mitzture of chametz is derived from כל מחמצת לא תאכלו. The Rambam as mentioned in 'Chametz as דבר שיש לו מתירין' holds that the chumrah of דבר שיש לו מתירין applies by chametz even to a מין בשאנו מינו due to the possuk of כל מחמצת. What does the Rambam see in this possuk about דשיל"מ?
Rav Leib Malen (siman 11) cites the Pri Chadash understands the Rambam rules that when it comes to chametz, it is assur if it כזית בכדי אכילת פרס is present even if there is no טעם of the issur. In light of this, says Rav Leib, we see that for chametz being מבטל the טעם doesn't suffice. The difference between מין במינו ושאינו מינו is that מין במינו there is no ביטול of the טעם and hence if it is a דשיל"מ it is more strict but for מבשא"מ when there is no more טעם the issur is gone entirely and the chumrah of דשיל"מ is not imposed for the issur is not in existence any more. But when it comes to chametz we see in the law of כל מחמצת that even without טעם the issur remains intact so the rule of דשיל"מ will apply even for מין בשאינו מינו. This works according to the peshat of the Pri Chadash. According to above peshat in the Rambam that the Rambam is referring to a regular case of כזית בכא"פ we would have to say that Rambam means is that even though the taste of chametz should not be assur, only the cheftzah of chametz itself, nonetheless כל מחמצת teaches us it is assur. So too, when it comes to דשיל"מ, the bar of bittul is raised. It is not enough to eradicate the shem chametz but any trace of chametz in a taaroves remains forbidden.