Why has ping started to resolve hostnames to IPv6 addresses

Windows has Connection profiles like "Private", and "Guest or Public" which manages if you can ping windows, or not.

You can see which profile is enabled for you with:

Control Panel\Network and Internet\Network and Sharing Center

and click "Advanced sharing settings"

There you see wheter the "Private" or the "Guest or Public" Profile is enabled.
Beside the enabled Profile name is (current profile), that marks it as active.
Inside the Profile enable "File and Printer sharing". And click "Save changes"

The "Guest or Public" profile is set to not respond to ping per default.
The "Private" profile responds to the ping by default.

To set the Connection Profile with powershell (in elevated mode) to Private use:

Set-NetConnectionProfile -InterfaceAlias "*" -NetworkCategory Private

there are a few things I would recommend you try to see if you can fix this:

1. Disable windows firewall if you have not done so already. It is a fairly well documented "feature" of windows firewall that it randomly blocks packets even if an exception is made. Look on Technet and you will see what I am on about!

2. Disable IPv6 on your NIC. If you are not using it, it will only cause problems! Especially in situations where both PCs speak ipv6 and the router/switch between them doesn't.

3. Reinstall your network adapter drivers. Intermittent connectivity cal often be caused by faultering/failing driver installs. Even if it doesn't fix the problem, it definitely won't hurt!

I have often found that the main reason this happens is one of the above... hopefully this will help set you on your way.