And the second is less of a possible bug than it is design choice, but I have found the lock screen to have low contrast against certain backgrounds, which could make it hard to read for some:
It might be really related to SWT. Eventually, if this does not happen with Adwaita, this could be related to how Yaru draws the window controls (https://github.com/ubuntu/yaru/blob/master/gtk/src/default/gtk-3.0/_tweaks.scss#L10). On the other hand this issue is new and we didn’t change the way we draw window controls for 4 years so maybe it is related to a gtk3 minor version that changed and SWT was somehow not flexible enough to any gtk3 changes.