Dollar sign from variable name swallowed in double quoted Strings

@prettier/plugin-php v0.19.4
Playground link

Input:

<?php

# simple syntax
# OK
"$foo[$index_var]";

# simple syntax with explicit name boundaries with curly brackets
# Not OK: prettier swallows the dollar sign from $index_var
"${foo[$index_var]}";

# complex (curly) syntax
# OK
"{$foo[$index_var]}";

Output:

<?php

# simple syntax
# OK
"$foo[$index_var]";

# simple syntax with explicit name boundaries with curly brackets
# Not OK: prettier swallows the dollar sign from $index_var
"${foo[index_var]}";

# complex (curly) syntax
# OK
"{$foo[$index_var]}";

In a double quoted string using the so called simple syntax with explicit variable boundaries given by curly brackets prettier swallows the dollar sign from a variable nested inside the boundaries.

Start with the linked Playground reproduction and compare the input and output for the explicitly bounded variable syntax inside a double-quoted string. Trace how that interpolation is formatted, then verify that the nested $index_var keeps its dollar sign while the other two examples remain unchanged.