Missing { inserted in \newcommand across pdfLaTeX, XeLaTeX, and LuaLaTeX

The goal is to determine whether the LaTeX2e error "Missing { inserted" appears at the macro definition site or is deferred to input-encoding resolution when the same \newcommand source is compiled with pdfLaTeX, XeLaTeX, and LuaLaTeX.

The kernel message is generic and does not distinguish a missing opening brace from a missing closing brace. Engine-specific parsing order may also change the reported line or context, so the exact component responsible for the deferred behavior is unclear without a controlled comparison.

• Does pdfLaTeX report the error at the definition point while XeLaTeX and LuaLaTeX defer it?
• Can the generic "Missing { inserted" message reliably indicate whether the missing token is an opening or closing brace?
• Which engine-specific layer controls the deferral behavior for macro arguments?