I found a better explanation from this site:
http://www.ncl.ac.uk/dental/oralbiol...eformation.htm
"High protein diet
Plaque formed on the teeth of individuals with a low carbohydrate, high protein diet, contains fewer acidogenic-aciduric organisms. The pH gradient will be different and the overall pH of the plaque alkaline because of the ammonia produced as a by-product of amino acid breakdown. The higher pH of the plaque will itself inhibit acidogenesis and favour Gram negative organisms which will be present in greater numbers. The proteolytic nature of the plaque will result in the presence of particular peptides such as putrescene and cadaverine which have a characteristic offensive odour."
The ADA stuff on plaque just states it is caused only from too much sugar, which gives credence to the first statement I posted, which was from a dental hygienist, and kind of suggested it was the lactose (milk sugar) from the protein drink.
Part of the design of IP is more alkaline foods to bring our pH back where it should be. It would seem during the initial shock of going from what we ate before to what we eat on Phase 1 IP, the organisms (yeah, gross) in our mouths change from maybe what we are used to feeling or tasting, to this new alkaline taste. This would also explain why it isn't necessarily the protein, it is actually the vegetables (a lesser credible site suggested oxilates in spinach) or just lack of the normal over-sugar environment. Another lesser credible site suggests that the metallic taste is a candida (yeast) die off symptom, and an alkalizing diet is used to cause yeast overgrowth to die off.
Also, this is an intentionally unbalanced diet, so it would make sense that over alkalinizing would have some symptoms that would lesson once eating becomes more balanced (70-60% alkaline, 30-40% acid).
Definitely a real thing.