Healers in League

So I've noticed something lately (mostly because I haven't happened to play my healing characters unless they were or I rolled them in ARAM)...

My healers seem to be healing for less the more MR my allies had. This was rather infuriating to realize, as unless I was or , my healing was already pretty sad. Even was having this problem, and her healing is a tad underwhelming to me considering the cost.

The example I have of this was, as , my total healing was supposed to be in the neighborhood of 240 HP for both myself and the target, but was instead healing my allies for around 160-185, and it seemed to correlate to their MR, the more MR, the less healing they received.

This really cuts into my normally aggressive Battle Priest mentality as a healer, and as I hadn't noticed anything about it before and didn't recall MR affecting healing previously, I was quite distraught. Is this intentional, or a possible bug?

...really hoping it's a bug.

Edit: No, they did not have grievous wounds of any sort at the time of healing, nor any dots.