The difference is clear, investing what you can afford is used when you are interested in building future wealth while investing what you can afford to lose applies to cases where you just want a short-term profit and it mostly happens when you're trying things out.
I have also often wondered why I hear in this forum that we should invest what we can afford to loose in Bitcoin, it makes it to seem as if investing in Bitcoin is a gamble. I know that it is when you are investing in a project that you are considering risky that you will use money that you can afford to loose so that if the project crashes, that your lose will be minimal. Bitcoin is an asset that is a store of value, just like gold and real estate, they are assets that we believe that they will outlive us and that it is safe to invest more than what we can afford to loose when investing in them.
When you want to gamble your funds in projects that you are not sure whether they will be profitable, that is when it is advisable to invest the amount that you can afford to loose. In cryptocurrency, investing in most altcoins like memecoins are risky, you are not sure whether the project that you want to invest in will be profitable or a scam, so it will be important to invest the amount that you can afford to loose. Maybe it is just a figure of speech in the forum otherwise I don't think that the phrase should be specifically used for Bitcoin investment.