Imperative Bob needs to fix a bug in Functional Dave's code. Or just call it from his imperative code with an appropriate data structure. Or write a module for Functional Dave to call.

All of those require being familiar with the functional paradigm, or a data type conversion. And vice versa.

Sure, for code that doesn't interact, or that does only through an interface not defined by the language (i.e., through the OS, through sockets, whatever), the issues in style don't exist, and Scala kinda makes sense as a "single language that tries to be all things to all people". But on a shared codebase, with a 'team style', per the OP, that allows people to use what they're most familiar with...mixing paradigms is going to cause problems, and will force each dev to learn each paradigm in use.

Please note I'm not saying there's no benefit to mixing paradigms, or that a language that includes multiple paradigms is bad, but rather that by allowing more paradigms the codebase you increase the breadth of knowledge necessary to work on that codebase, such that letting people stay in their comfortable, single paradigm bubble is impossible, at least if you want them directly interacting with the code written by someone else in a different paradigm.