Hi there,
There is no need for concern, this is very normal. Your browser is caching the parts of the map, this is normal and used to reduce load on the server. usually you can force a full refresh with Ctrl + Shift + R in most browsers. The cache will also naturally expire over time. You can read more about it here: https://wiki.openstreetmap.org/wiki/Browser_cache
Kind regards,
Kits