Powershell is failing bamboo task because -ExecutionPolicy bypass -Command return code is 134 instead of 0
I've had a cursory Google of the issue and it looks like this is actually a bug with PowerShell for Linux (and MacOS by proxy), at least according to the PowerShell for Mac GitHub issue I managed to find for it. The user who reported the issue with PowerShell seems to have a very similar issue with the -ExecutionPolicy parameter when calling a PowerShell script from a Linux distro (Ubuntu?) and I would assume that this would extrapolate out to MacOS due to the UNIX architecture.
From the looks of the comments that were added, this seems to have been fixed in Alpha Build 18. If this is still not working on MacOS with the latest build of PowerShell, I might suggest you report it on the PowerShell GitHub like this user did :)