If you see grid lines or gray thick lines inside your map, like the example below, this is likely a simple conflict with your theme that can be easily overridden and fixed.
Copy and paste the following code into your Design Settings:
.leaflet-tile-container img {
margin: 0 !important;
padding: 0 !important;
}
Save your changes. You may need to clear cache on your website, if you are using caching plugins or host level caching, before this change takes effect. You can also load an incognito browser window to check the update.