For some time (please apologize being vague) in the nightlies/development versions connecting ways can downgrade the previously built way.
For example lets assume a 100km/h road gets connected to a 70km/h road, then the 100km/h road gets downgraded to 70km/h at the resulting junction.
I remember this being not the case, i.e. the junction would stay being a 100km/h road.
This happens with railways too.
You sure you do not have a stuck key? This happens if you hold "Ctrl" when building a way since that allows you to downgrade ways.
Thank you! This exactly what I did. Now can understand why this happening.
That is a bit of conflicting keys. Ctrl is also used to change the auto-build strategy. It's impossible to use the alternate strategy without downgrading the ways you cross. This is unfortunate, although normally I use Ctrl to keep auto-building from routing over existing ways.