OS? Monitor DPI?
This alt-backspace is new to me.
Is this different from the ctrl-z behaviour?
Apologies for omissions.
Windows 10 Pro; Version 1909 (Build 18363.752);
Monitor 1: 3840x2160 at recommended 150%.
Monitor 2: 2560x1440 at recommended 100%.

Now slightly embarrassed - did a clean install of Code::Blocks and both issues disappeared. It must have been something in the AppData folder (that has carried over cleanly without issue for quite some time - and lulled me into continuing bad practice) that changed. Apologies for the false reports.