Smooth CSS transition from position relative to position absolute with :hover::after - html

Question

I am trying to build an underline under the nav menu on hover. I'm using the 'content' trick because I couldn't make what I wanted with just text decoration.
I want something like this but with a smooth transition.
Here's the CSS for it without transition because I couldn't make it work.
div {
display: flex;
align-items: center;
color: #ab9b8c;
font-size: 20px;
letter-spacing: 1.5px;
text-decoration: none;
height: 100%;
cursor: pointer;
transition: height 2s;
position: relative;
}
div:hover::after {
content: '';
position: absolute;
bottom: -1px;
width: 30px;
height: 3px;
border-radius: 200px;
border: 1px solid #9f9182;
}
<div>test
<div/>

Answer 1

As Paulie_D mentioned, you can't do transitions on position attributes. See: https://www.w3schools.com/cssref/pr_class_position.asp "Animatable: no."
Several other attributes are fair game though. Here's one using the width and the border:
div {
display: flex;
align-items: center;
color: #ab9b8c;
font-size: 20px;
letter-spacing: 1.5px;
text-decoration: none;
height: 100%;
cursor: pointer;
transition: height 2s;
position: relative;
}
div::after {
content: '';
position: absolute;
bottom: -1px;
width: 0px;
height: 3px;
border-radius: 200px;
border: 0px solid #9f9182;
transition: all 0.2s linear;
}
div:hover::after {
width: 30px;
border: 1px solid #9f9182;
transition: all 0.2s linear;
}
<div>test
</div>
Here's an opacity:
div {
display: flex;
align-items: center;
color: #ab9b8c;
font-size: 20px;
letter-spacing: 1.5px;
text-decoration: none;
height: 100%;
cursor: pointer;
transition: height 2s;
position: relative;
}
div::after {
content: '';
position: absolute;
bottom: -1px;
width: 30px;
height: 3px;
border-radius: 200px;
border: 1px solid #9f9182;
opacity: 0;
transition: all 0.2s linear;
}
div:hover::after {
opacity: 1;
transition: all 0.2s linear;
}
<div>test
</div>
Here's both at once because why not:
div {
display: flex;
align-items: center;
color: #ab9b8c;
font-size: 20px;
letter-spacing: 1.5px;
text-decoration: none;
height: 100%;
cursor: pointer;
transition: height 2s;
position: relative;
}
div::after {
content: '';
position: absolute;
bottom: -1px;
width: 0px;
height: 3px;
border-radius: 200px;
border: 0px solid #9f9182;
opacity: 0;
transition: all 0.2s linear;
}
div:hover::after {
width: 30px;
opacity: 1;
border: 1px solid #9f9182;
transition: all 0.2s linear;
}
<div>test
</div>
So on and so on