The experience varies by browser and device. On Firefox desktop, having accepted the warning and set an exception, it doesn't warn me again. The crossed-out padlock shows the exception still applies, so the certificate is still invalid.

On Firefox Android the warning is repeated if I close then reopen the browser (rarely) or it's unloaded to free memory for another app (occasionally) and then I load Mudcat. So it's more of a nuisance. Again, there's a crossed-out padlock.

Other browsers may make life even more difficult.