I know this is an old question but for anyone looking, one solution may be to clear your cached images and data in your browser.
This has been particularly prevalent for me on Google Chrome, especially if any CSS changes were made.
On Chrome (Windows):
Hit Ctrl+Shift+Delete to bring up the "Delete Browsing Data..." menu.
Select only the "Cached images and files" checkbox.
Press "Delete data"
You may need to refresh the page, but the bulk of the time I've encountered this issue working with Blazor this has remedied the issue.