Solution to my problem. Adding this here should others have the same problem:
On an old site, I have a frontend built using vue.js and lodash because older browsers don’t quite support the good stuff.
I scoped lodash to _u = _.noConflict(). I always do this to protect against conflicts with other plugins etc.
Recently, the frontend stopped working… lodash was undefined. Viewing source I see that Beaver Builder now automatically scopes lodash - sticking it on the window object… it does this just after the lodash is loaded, and BEFORE my _u (hence this broke - as _ was redefined before I scoped it)
Beaver builder adds:
window.lodash = _.noConflict();
So I went through and updated all my frontend scripts, replacing _u with window.lodash - and stuff now works…