> ...they just happened to have a prebuilt UI component

With respect, that's not what is described. OP rolled an API, handed documentation to frontend, and "after a week" frontend "came up" with a UI that held different expectations for how pagination works than the API actually worked. I'm being charitable in assuming that there was a good reason that this pagesize, offset pagination style could not work, but as written, there was no communication about that, neither to us the audience nor to OP's colleagues. In fact, frontend apparently expected this.

But why not discuss offset, pagination with colleagues? Why just present it as fait accompli and move on to create a library that further locks it in?