Beaver Builder Theme is built on the Bootstrap framework, and Bootstrap is transitioning from Version 3 to Version 4. Here's a good technical summary of the changes in Bootstrap 4. Note in particular the change in browser support.
In Customize > General > Layout, there's a Framework field with settings to help in the eventual transition to Bootstrap 4 or to choose which version of Bootstrap you want if you're having any issues. You can choose from the following settings:
- Minimal Bootstrap 3
The default setting, and the one to keep if you're not sure. It loads a minimal version of Bootstrap 3 with the functionality required to ensure that the theme works as it does now.
- Minimal Bootstrap 4
Loads a minimal version of Bootstrap 4.
- Full Bootstrap 3
Loads the full version of Bootstrap 3.
- Full Bootstrap 4
Loads the full version of Bootstrap 4.
If you have a new site and want to use Bootstrap 4 features and don't mind the loss of support for older systems, you can try choosing Bootstrap 4. If you have existing customizations using Bootstrap 3 or Bootstrap 4 and need to ensure your layouts are not broken, choose the appropriate version.