Or HaChaim on Genesis 40:23/אור החיים על בראשית מ׳:כ״ג

1 ולא זכר שר המשקים את יוסף, and the chief butler did not remember Joseph, etc. This means that although he had not yet forgotten him, he did not remember him either. At the beginning he simply did not recall Joseph's name, something that Joseph had asked him to remember.

2This verse also informs us that the chief butler subsequently forgot Joseph completely, he erased the incident from his heart. The Torah indicates that once one has decided not to remember something or somebody such a memory can be blocked out completely. Unless the chief butler had made a conscious effort to blot Joseph from his mind he would have remembered the incident from time to time. Perhaps the Torah wrote וישכחהו, he forgot him, in order to hint that this was a deliberate act of forgetting. Alternatively, as long as G'd did not remember Joseph's condition there was no point in the chief butler remembering him either. It could also be that the word וישכחהו means that only he forgot Joseph whereas G'd most certainly did not forget him.