Does freezing sap freshness? Will acids bring the flavor back? I just dethawed a frozen lentil soup which was poorly seasoned before it went into the freezer. I added salt to the other half which I ate fresh and made a mental note of seasoning the frozen batch once thawed. Once I thawed it, I corrected the seasoning and observed there was this unpleasant metallic flavor to the soup. I can't put a point on it, but it tasted "off". 

I considered for a moment: what if extreme temperatures sap the freshness out of food by reducing the acidity of the fresh product? Lo, when I gave it a slight splash of sherry vinegar, all was right again. However, I love sherry vinegar and use it often in any salad or soup to sour it up to my taste.

My question is: does freezing reduce acidity in products? Is acidity what we perceive to be the "fresh" flavor of things not previously frozen or boiled to oblivion?
Freezing will not reduce acid.   Chemical reactions tend to slow down when at freezer temperatures, not speed up.

Fresh flavor is a complex amalgam of many factors:  texture, aromatics, color, and so forth.   While acid is perceived as lending a brightness, it is not the only factor in freshness.  After all, your sherry vinegar is far from fresh, but quite acidic.

While there may have been something off in your stored portion that was masked by adding additional vinegar, it is almost certainly not related to having frozen a portion of the soup.