Try to add the following code snippet to the Custom CSS area in Theme Settings to fix this issue.

.woocommerce-checkout .checkout {
	margin-left: -15px;
	margin-right: -15px;
  -webkit-box-align: flex-start;
  -ms-flex-align: flex-start;
  align-items: flex-start;
}

.woocommerce-checkout .checkout > div {
	-ms-flex-preferred-size: 50%;
	flex-basis: 50%;
	max-width: 50%;
	width: 50%;
	padding-left: 15px;
	padding-right: 15px;
}

.woocommerce-checkout .checkout .col2-set {
    -ms-flex-direction: column;
    flex-direction: column;
}

.woocommerce-checkout .checkout .woocommerce-checkout-review-order {
		position: relative;
    background-color: #f8f8f8;
	  padding: 30px;
	  margin-left: 15px;
    margin-right: 15px;
}

.woocommerce-checkout .woocommerce-checkout-review-order:before {
	content: "Deine Bestellung";
	font-size: 22px;
  font-weight: 600;
  text-transform: uppercase;
	color: #2d2a2a;
	display: block;
	margin-bottom: 20px;
	text-align: center;
}

.woocommerce-checkout #order_review_heading {
	display: none;
}

@media (max-width: 768px) {
.woocommerce-checkout .checkout {
	margin-left: 0px;
	margin-right: 0px;
}
	
.woocommerce-checkout .checkout > div {
	-ms-flex-preferred-size: 100%;
	flex-basis: 100%;
	max-width: 100%;
	width: 100%;
	padding-left: 0;
	padding-right: 0;
}
	
.woocommerce-checkout .checkout .woocommerce-checkout-review-order {
	  margin-left: 0;
    margin-right: 0;
}
	
}

Regards