"failed to synthesize GetElem?" with `of_getElem?_eq_some`

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

I get the above error when I try to use of_getElem?_eq_some. Curiously, using a temporary variable eliminates the error.

Steps to Reproduce

example (l : List String) (i : Nat) (h : l[i]? = some "") : i < l.length :=
  of_getElem?_eq_some h
/-
failed to synthesize
  GetElem? Nat Nat ?m.14 LT.lt
-/

example (l : List String) (i : Nat) (h : l[i]? = some "") : i < l.length :=
  let dummy := of_getElem?_eq_some h
  dummy
/- success -/

Expected behavior: No error.

Actual behavior: Error.

Versions

Lean 4.23.0, 4.25.0-nightly-2025-09-26

Start by running the two minimal examples from the issue on Lean 4.23.0 and the referenced nightly, then compare elaboration of the direct and let-bound forms. Done means the direct use of of_getElem?_eq_some elaborates successfully, with a regression test covering the reproducer.