very interesting. thanks for the update.
and i must add that if the problem is in fact caused by
DNS cache, it is worth knowing that
Google Chrome is also storing DNS separately here: chrome://net-internals/#dns
and that also might need a flush.
here is an article:
https://www.howtogeek.com/197804/how-to-clear-the-google-chrome-dns-cache-on-windows/i have been affected by chrome's DNS once or twice in the past for other sites!
i don't know of any other browser that stores "DNS" internally.