I also have a daily lunch blocker in my calender. I'm somewhat flexible with my lunch break but that's for me to decide.

My advice: Treat your lunch break as any other meeting that you have in your calender.

If someone schedules a meeting into your break, I'd except them to reach out to you and try to ask if the meeting would be possible.
Since you are in software development, I expect that you have some sort of daily standup where such meetings on a short nice could also be adressed from your manager, like "Hey, I need to talk to you today regarding X". 

When you are invited to meetings with just a couple of hours in advance, there's also the chance that you didn't see the invitation e-mail in time.