Why is align-self property in flexbox not working?

Question

I am really new to this, and I have no idea why the 6th .flextest stays with the other .flextest and not the other end. any help would be appreciated. (Section3 is the container of the flextests)

.flextest:nth-of-type(5) {
    width: 100px;
    height: 100px;
    background-color: rgb(0, 36, 36);
    margin-top: 50px; 
    color: azure;
}

.flextest:nth-of-type(6) {
    width: 100px;
    height: 100px;
    background-color: rgb(0, 0, 0);
    margin-top: 50px; 
    color: azure;
    display: flex;
    align-self: flex-end;
}

.section3 {
    display: flex;
    /*justify-content: space-between;*/
}