You're multiplying px by px, which is a "square px" or px².
CSS doesn't have area units so your code is invalid.

The correct code should have just one px:

  letter-spacing: calc(21.72px * 7.059);

The values are generated by a lib, I can't simply leave off the "px" from one of the values.

I can't find anything in the spec
  https://www.w3.org/TR/css-values-3/#calc-notation
that says the browsers can must not resolve
  calc(5px * 10px)
to
  50px
.

The resulting value can't be in square-pixels anyways (since as you note CSS doesn't have area units), so I think returning 50px would not be an issue at all.

The relevant section is https://www.w3.org/TR/css-values-3/#calc-type-checking:

At *, check that at least one side is <number>...

I don't think it makes sense to have unit * unit = unit; that would break a logical interpretation of the statement.

> so I think returning 50px would not be an issue at all.

Apart from violating the specification that would also violate basic mathematics/calculus.
It would be also wrong to have a mathematically invalid exception just for the same-unit case.
It would be even more wrong to coerce to whichever unit is the first/last one in case of mixed units.

The status got set to WontFix and I hadn't commented since that 😀
It's fine.