It turned out to be Actor Snapping.
I recall hitting the “End” key a few weeks ago trying to achieve a one-time actor snap, but it enabled it permanently.
In UE4: Edit > Editor Preferences > Viewports > Enable Actor Snap
You may find that “Actor Snapping” is enabled. I couldn’t directly disable mine. I had to click “Reset to Defaults” at the top.
I hope this helps.