The two JS functions below are identical except for the const variable name length. They produce different output - even when mangled:

$ echo 'function f() {const a = "X"; return a + a;}' | uglifyjs -cm
function f(){const n="X";return n+n}

$ echo 'function f() {const aa = "X"; return aa + aa;}' | uglifyjs -cm
function f(){return"XX"}

Running compress before mangle in bin/uglifyjs may have something to do with this. Either that or compress needs a better heuristic governing when const substitution should take place.