The travis change of default dist may be related.
Looks like change of default distro from trusty to xenial got rolled out gradually, and if you didn’t specify dist, your default dist changed when it got rolled out, from trusty to something else, which made existing config stop working, but telling it to use trusty again made it work again?
Doens’t explain @pgp saying he saw things claiming to be xenial with different version numbers, that could be a different bug?
Hard to say.