Are Humble Monthly games added to the bundled games list?

oh i guess not all get added to the list?

Pretty much (which is to say, most of the time but not necessarily always). Searches for Dropsy and Chroma Squad have thrown up no matches, while as ambidot has stated, Alien Isolation and Talos Principle were added to the list due to different reasons.

A:I seems to be due to Monthly (other threads have nixed Russian pricing, and the timing of the bundling matched Humble Monthly's launch of it), Talos I believe was due to Russian prices.

At the very least, A:I would be the only example of a monthly that might be bundled because of monthly.

So, in summation, you definitely shouldn't usually have to worry about monthly games being added to the list- even if they are being added from monthly, they'd have to have very high retail to qualify for it, and most humble monthly games tend toward $10-20 retail, from what I've seen.
So, likely, you're probably not going to see more than one a month get marked, as a general guideline.