bootstrap 4 loses custom css on small screens

In bootstrap 4.3.1 the style is lost in small screens. This was not a problem with 4.2.1. In the width of 576px and smaller, the coding of the divider strip is plain text with no lower edge. The custom css is:

.divider-strip {
min-height: 1px;
top margin: 20px;
bottom margin: 20px;
screen: online block;
relative position;
bottom edge: 1px # 607d8b solid;
}
.divider-strip h4 {
weight of the source: 700;
bottom margin: 10px;
text transformation: none;
}
.strip-block {
width: 100px;
height: 3px;
background: # 607d8b;
position: absolute;
down: -2px;
left: 0;
}
.block-title {
width: 100%;

The html is:

Accordion 1

Please, tell me what I need to fix in this. Thanks in advance.