Firebase projects not compiling with UCB

I have two separate UCB projects, both of which are failing to build for iOS on UCB with the latest version of Firebase's Unity Plugin (5.4.0) with the error message:

40064: ▸ ❌; Undefined symbols for architecture arm64
40065: ▸ > Symbol: _OBJC_CLASS_$_SFSafariViewController
40066: ▸ > Referenced from: objc-class-ref in FIRAuthURLPresenter.o
40067: ▸ ❌; ld: symbol(s) not found for architecture arm64
40068: ▸ ❌; clang: error: linker command failed with exit code 1 (use -v to see invocation)

Both projects are building fine for iOS locally, although I should add that I'm using XCode10 locally, whereas UCB is still locked to XCode 9.4.1. I don't actually have an install of XCode 9.4.1 to reproduce the issue with, although I suspect it will build locally (more accurately, I know firebase was working locally with 9.4.1 recently.)

I realize that this question kinda straddles UCB and firebase, so I have to pick a forum to ask it on rather than double posting, but are there any known issues with the cocoapods support in UCB at the moment, as I suspect this will be the reason.

The fact that I see that same error in two completely different projects, both of which are fine locally, makes me suspect that the problem can be reproduced with a UCB build of any Firebase project, with is worrying as a lot of apps make use of Firebase.

Any ideas?