TP-Link has made available new firmware updates for most models that resolves the issue. The update changes their internet connectivity testing from NTP requests against hardcoded addresses including NTP pool servers to ICMP requests against the local network gateway.
Notably, these devices don’t have auto-update, the manual check-for-update button in their web interface always reports that the software is up-to-date, and TP-Link doesn’t make the latest firmware versions available through their websites in all countries. E.g. updates for the NTP issue isn’t available for most models on TP-Link’s US website.
In other words, these devices are very unlikely to ever receive any updates that fixes this issue.