Welcome to the Community,
yes, you are right, this seems to be a bug.
Anyway, to work around this - after selecting a datastore with free disk space - either toggle thick/thin provisioning, or temporarily specify a thick disk size that's larger than the free disk space (e.g. make GB -> TB). One of these options should enable the Save button again. Then configure the settings as needed, and save the settings.
Thanks for the welcome and response. I tried all of that (those were actually the first things I tried). I could never get the "Save" button to re-enable again after it disabled initially. Additionally, even deleting the new disk altogether from the VM configuration (again, in "Edit" of the web client), the "Save" would never re-enable.
I'm guessing this hasn't been found sooner since any enterprise customer would be using vSphere and this problem is unique to the ESXi host's own web client.
I actually just tried these workaround before responding, and it worked for me.
Client version: 1.30.0
Client build number: 9946814
ESXi version: 6.7.0
ESXi build number: 10302608
Interesting, I had done the provision changes and had even tried changing the size of the partition (but all within the available space on the datastore) and "Save" never re-enabled- it still doesn't. However, when I add a new disk (from scratch) and select a size over what's available and then drop it back down to what I actually want, "Save" re-enables just like you said! So definitely a bug, but another viable workaround.
I understand VMware probably has bigger problems on their hands right now with the recent release of 6.7U3, but is there anything else I need to do from here to more formally report it to them?
To report issues officially, you'll need to open a support case with VMware.
However,for the Embedded Host client there's another option. You can report a bug at the ESXi Embedded Host Client's Flings page.