IME if you're below 60mg (I'm assuming its 1mg/mL?? otherwise theres no way to quantify how much methadone the OP is taking), Methadone does not at all block strong opiates like heroin, especially when used intravenously. It has also been my experience that if you take decent break from the heroin and just use the methadone at a below blocking dosage, that yes, you will get way more fucked up off of the heroin (if you take the heroin after your methadone). In fact this can be quite dangerous and used to fuck me up royally. Yes Methadone is a strong opioid but 45mg is not a very high dosage, and in fact a few months ago when I was held up in a hospital for sepsis, even 60 mg barely took away my withdrawal symptoms from the BTH (which I strangely find much harder to withdraw from than NY/NJ ECP-I believe it has to due with the cruder synthesis allowing more alkaloids that aren't necessarily binding to opiate receptors being included in the final product).
Also, remember that while cross tolerant, methadone and heroin are metabolized very differently and are also administered differently (in fact heroin is a prodrug, just a faster delivery system for morphine to burst through your blood brain barrier). This also plays a role in how one can still get high off of heroin while on methadone.