A standard LaTeX interpreter will not add a space between $\delta$ and $a$.

This is not a "space". It is multiplication.

LaTeX does not understand the meaning of formulae and has no concept of multiplication. LaTeX only does typesetting. LaTeX code is not appropriate for representing precise mathematical meaning.

`ToExpression`

needs to convert it to a meaningful mathematical expression. It has to decide whether you meant one symbol named $\delta\alpha$ or the multiplication of two distinct symbols, $\delta\times\alpha$. `ToExpression`

does not preserve typesetting. It makes a guess about the mathematical meaning and gives you the corresponding expression, which can be typeset in multiple different ways.

The guesses is makes are not always good. I believe the idea is that you will correct them manually. So I suggest just fixing the output by hand.

If you really need to get this output automatically, a possible solution is to convert the LaTeX to MathML using some other, more configurable tool. Then import the MathML to Mathematica. I would try this if I had to process hundreds of such inputs.