If I delete a program section in the "block" section, it also deletes the corresponding button. This is very annoying. I created a 200-block running program section, then accidentally duplicated it. I deleted the excess, but the program deleted both the original program section and the button. I lost 2 hours of work.
The behaviour you describe is a current bug, being worked on.
Undo or CTRL+Z should restore your deleted blocks though...