For future reference, just assume we need more information than what you think is sufficient. 
Your original title made no reference to AUR PKGBUILDs (until it was later edited after discovering this was only an issue with AUR/PKGBUILDs; and not an issue with “updating your system”.)
This goes away from the standard Manjaro system, so it’s another important detail to mention in the original post. An agnostic package manager, not used by default in a Manjaro system, is kind of important to bring up when you’re facing issues updating your system through its package manager.
I’m not trying to sound rude, but this seems to be a common pattern in this forum. Not providing relevant and important information is not simply an issue of “not enough information”, but it also increases the chance that other users (who want to help you) will go off on wild goose chases and be led astray by red herrings, since every problem has an infinite number of causes without the proper context.
Imagine someone creating a thread titled “Can’t update my Manjaro”. Yet they don’t mention they’re behind a VPN or proxy, and they’re using a USB-to-RJ45 adapter. Then they later also mention “Oh and I can’t seem to visit certain websites. Some websites work, but others don’t.” So now we know it’s not really a problem with “updating Manjaro”.