Нужно чтоб они были одной высоты. Т.к дивы обернуты в блок слайда, флексовое распределение высоты не работает.
<div class="swiper">
<div class="swiper-wrapper">
<div class="swiper-slide">
<div class="reviews__block">
<img class="reviews__block-img" src="img/avatar-1.png" alt="">
<p class="reviews__block-text">текст</p>
<p class="reviews__block-text reviews__block-subtext">текст</p>
</div>
</div>
<div class="swiper-slide">
<div class="reviews__block">
<img class="reviews__block-img" src="img/avatar-2.png" alt="">
<p class="reviews__block-text">текст</p>
<p class="reviews__block-text reviews__block-subtext">текст</p>
</div>
</div>
<div class="swiper-slide">
<div class="reviews__block">
<img class="reviews__block-img" src="img/avatar-3.png" alt="">
<p class="reviews__block-text">текст</p>
<p class="reviews__block-text reviews__block-subtext">текст</p>
</div>
</div>
CSS
.swiper {
overflow-y: visible;
overflow-x: clip;
}
.reviews__block {
display: inline-block;
position: relative;
padding: 108px 46px 48px 48px;
border: 2px solid #314b26;
height: 100%;
}
.swiper-slide:nth-child(even) > .reviews__block{
background-image: url(../img/review-block-bg.jpg);
background-size: cover;
color: #fff;
}
.swiper-slide:nth-child(2n) .reviews__block-subtext{
color: rgba(255, 255, 255, 0.60);
}
.reviews__block-img {
position: absolute;
top: -80px;
left: 50px;
z-index: 1000;
border-radius: 160px;
border: 2px solid #314b26;
}
.reviews__block-text {
max-width: 446px;
min-height: 186px;
line-height: 30.6px;
}
.reviews__block-subtext {
margin-top: 20px;
min-height: auto;
color: #838383;
}