The evidence I'm aware of seems to show that acid forming diets may cause calcium to be leached from the bone to balance pH levels. So an important comment is that most people wont experience metabolic acidosis eating an "acid forming diet", they'll just lose some skeletal calcium to maintain their alkaline pH. So bone health is probably the most important health marker to look at when discussing acid forming diets, in my opinion. Which brings me to some studies...
In this study, increasing protein from low to moderate levels improved bone health.
In another study, in which protein intake was increased significantly, the authors stated "We conclude that increasing protein intake from 0.78 to 1.55 g/kg·d with meat supplements in combination with reducing carbohydrate intake did not alter urine calcium excretion". This and other studies discuss the importance of protein on bone health and mechanisms by which protein does not cause net calcium loss from the bone.
Here's a study titled "Controlled High Meat Diets Do Not Affect Calcium Retention or Indices of Bone Status in Healthy Postmenopausal Women". The title says it all.
In this study the authors stated "Under practical dietary conditions, increased dietary protein from animal sources was not detrimental to calcium balance or short-term indicators of bone health".
In this study in which protein intake was increased from 1 to 2.1 g/kg, the authors wrote "These data directly demonstrate that, at least in the short term, high-protein diets are not detrimental to bone".
This study, in which women increased protein intake from 61 to 118 grams a day, provides more evidence that "a high-protein diet has no adverse effects on bone health".
So ultimately I think animal protein may be acidifying, which leads to calcium loss, but it may also increase calcium absorption, negating the possible negative effects. And perhaps for a few reason, protein is a beneficial nutrient for bone health. I think eating a fair amount of fruits and vegetables is good, but as long as you're eating some and your "acid" comes from meat, pH balance isn't something I'd worry to much about.
*Much of this answer is copied from an answer I gave a few weeks ago, but the question was very similar.