Consider these examples:
MatchQ[, _] (* True *)
MatchQ[{wald}, {_Symbol}] (* True *)
MatchQ[5, _Integer] (* True *)
MatchQ[{wald}, _{_Symbol}] (* False *)
Why does Mathematica not see the fourth pattern as valid, namely as a pattern restricted to be a list consisting of a Symbol (or alternatively, as an empty expression followed by a list of a Symbol)?
I am using Mathematica 12.2.
One source of confusion here is about what MatchQ[, _] actually does. Contrary to what you think, the first argument of MatchQ here is not "the empty expression" (which isn't a thing in WL) but it's Null.
Consider, for example:
{, 1}
Hold[, 1]
{Null, 1}
Hold[Null, 1]
Null is a special value that doesn't get printed when it's returned to the FE as the result of an evaluation, but it will get printed if it's part of an expression:
Null (* no output *)
{Null}
{Null}
So MatchQ[, _]is just MatchQ[Null, _], which is obviously True.
The closest thing you get to an "empty expression" is Sequence[], which disappears whenever it is enclosed by a head that does not have the SequenceHold attribute:
f[Sequence[]]
HoldComplete[Sequence[]]
f[]
HoldComplete[Sequence[]]
but on its own it's still a non-empty expression:
Sequence[]
Sequence[]
Your pattern is actually interpreted as a multiplication:
FullForm[_{_Symbol}]
(* List[Times[Blank, Blank[Symbol]]] *)
To match a list consisting of a Symbol, use List[_Symbol] as a pattern:
MatchQ[{wald}, List[_Symbol]]
(* True *)
FullForm[_{_Symbol}]. I think it's different than what you think it is, maybe. $\endgroup$_[_Symbol]? $\endgroup$