Apparently, an update to VirtualBox after version 6.1.26 limited the IP’s usable for network adapters on Mac / Linux hosts. They must now be in the 192.68.56.0/21 range, which is pretty limited and much less easy to remember or type than the 10.*.*.* that I had been using. I had to change my projects to all be in this range and spread out the IPs to avoid collisions between the various projects when I updated VirtualBox a while back.
For some time after this, I was having a problem where the IPs would only (usually) successfully allocate the first vagrant up after a reboot. So I could work on one project, then would have to reboot if I wanted to work on another and needed more than just vagrant ssh. I updated to version 7 today and am glad to see that that problem is fixed: I can once again start and stop multiple projects at will without IP problems. I guess they have switched from using kext (kernel extensions) to using hypervisor and other APIs on Mac hosts, which is probably a good thing in general and hopefully have less security concerns.
I run my project virtual machines with Vagrant using VirtualBox and a network config like:
#==network
web.vm.network 'private_network', ip: '192.168.56.6'
#--connect to internet
#-@ https://stackoverflow.com/a/18457420/1139122
web.vm.provider 'virtualbox' do |vb|
vb.customize ['modifyvm', :id, '--natdnshostresolver1', 'on']
vb.customize ['modifyvm', :id, '--natdnsproxy1', 'on']
end