I understand why you are using h2 element.
WordPress post visual editor not have font size controll.
If you are, if you are using h2 in order to increase the font size, and should do as follows the style you want to paste
This way, without the need to use longer h2, it is possible to properly display
Please try below
Dashboard / Appearance / Customize
Advanced / Site-wide CSS
copy and paste below code at textarea
article .entry-content h6,
article .entry-content h5,
article .entry-content h4,
article .entry-content h3,
article .entry-content h2,
article .entry-content h1{
clear:both;
display:block;
}
article .entry-content h6:before,
article .entry-content h5:before,
article .entry-content h4:before,
article .entry-content h3:before,
article .entry-content h2:before,
article .entry-content h1:before{
content: "\a";
display:block;
height:1px;
}
.entry-content{
font-size:138.5%;
}