As mentioned in the previous replies, the issue appears to be Xcode 12's inclusion of arm64 arch in the iPhone simulator, thus causing a clash between iPhone and Simulator when they are being combined to create the fat library.
So I followed kai_kai and @chimcanhcutptit@gmail.com's recommendation below and appears to have solved the problem (I was able to run my library in both simulator and real device). Basically the solution is to exclude arm645 architecture from the iphone simulator. This could have an impact when testing on arm64 macs later on - or may be not.
Select the project or target -> Build Settings -> Excluded Architecture (if you can't see it make sure [All] properties are selected) and set the Excluded Architecture for 'Any iOS simulator SDK' -> 'arm64'.
martemas
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