The issue affects _all_ models (inclusive the "hidden" guardian codex-auto-review model), and _only_ happens on the PRO plans (both 5x and 20x). It does not, under any circumstances, happen with the PLUS plan.
There is a probably related difference of CTX windows being different (and significantly larger on PLUS) between the plans, as well as an ominous and not-user-controllable flag `use_responses_lite` which is set to true on PLUS. These two discrepancies between the plans may or may not be the cause for the issue.
Amending the base prompt shows some improvements using the canary test prompts, NOT so during real world tasks however.
The API presents mixed results: from zero failures to 2/5 failures across runs.
These results have been taken from deterministic tests and analysis of historic data on both PLUS, PRO and API accounts. More details visible on the related GH issue in comments.