Hello jemma84,
Thank you for using Apple Support Communities. It's my understanding that the Side button doesn't consistently lock your iPhone 11 after recently updating iOS. I can see how this could be concerning. I'm happy to help.
I recommend reinstalling iOS with a computer. This step can resolve possible software issues by removing your current software and installing a fresh version. While your computer will try to reinstall the software without erasing your data, it's definitely a good idea to make sure that your content is backed up prior to trying this step.
How to back up your iPhone, iPad, and iPod touch
Use the steps in the link below to reinstall iOS:
If you can't update or restore your iPhone, iPad, or iPod touch
If the issue persists, please follow the remaining steps in the following resource:
If a button or switch isn't working on your iPhone or iPad
Have a nice day!