Snippet expansion is not considered as function node reevaluation-trigger

[micangl]

I have the following snippets:

local f1 = function(args, parent)
  if string.match(args[1][1], [[\frac]]) then
    return [[\left]]
  else
    return ""
  end
end
local f2 = function(args, parent)
  if string.match(args[1][1], [[\frac]]) then
    return [[\right]]
  else
    return ""
  end
end
s({trig = "()", priority = 1000, condition = in_mathzone, wordTrig = false, snippetType = "autosnippet"}, fmta([[<>(<><>)]], {f(f1, {1}), i(1), f(f2, {1})}))

s({trig = "//", dscr = "Fraction.", condition = in_mathzone, wordTrig = false, snippetType = "autosnippet"}, fmta([[\frac{<>}{<>}]], {i(1), i(2)}))

Basically, when I type () the cursor is positioned inside the parentheses. Then, if I type the \frac sequence, the parentheses are prepended, respectively, by the \left and \right directives; like this: \left(\right).

To insert the fraction directive \frac{}{} I have the // snippet. Unfortunately, when I type // inside the parentheses and the snippet is correctly expanded to \frac{}{}, the \left and \right directives don't appear immediately; I have to type something for luasnip to insert them.

I have read in the documentation that the function nodes are evaluated each time one of their arguments changes. Clearly, in this specific situation, this isn't happening.