Separation of data and algorithm is so useful. I can't really comment on how your senior is doing it, but in the area of numeric calculations, making numbers know anything about their calcs is a Bad Idea. Even associations with their units or other metadata should be loose. Functional programming provides such a useful intellectual toolkit even if you program in Java.
Sorry to learn, hope you don't get scar tissue from it.
Not sure how many people are writing programs with lots of numeric calculations.
Most programs in my experience are about manipulating records: retrieve something from a database, manipulate it a bit (change values), update it back.
Over here OOP do a good job - you create the data structures that you need to manipulate, but create the exact interface to effect the changes in a way that respect the domain rules.
I do get that this isn't every domain out there and _no size fits all_, but I don't get the OP complaints.
I currently think that most of the anger about OOP is either related to bad practices (overusing) or to lack of knowledge from newcomers. OOP is a tool like any other and can be used wrong.
Creating good reusable abstractions is not easy. It's quite possible to create tarballs of unusuable or overwrought abstractions. That is less of a knock on OOP and more a knock on the developers.
But that is what classes does, it lets you have data lists and dictionaries implemented as a class so that your algorithm doesn't have to understand how the data structure is implemented. In functional programming the algorithm has to be aware of the data structure, I feel that is much worse.