is there any chance the move to ARM will positively impact the feature request for deploying to iOS? (perhaps Raspberry Pi as well?)
Since both use ARM processors, yes — though the majority of the work will remain to be done (user interface APIs/conventions, I/O APIs, windowing system APIs, Unix API differences, and for iOS/iPadOS the restricted runtime environment and App Store review requirements).
I am mostly worried it will negatively impact a possible port to a Windows version in the future though.
It won't be helpful toward Windows support for the majority of Windows machines, which have x86_64 processors. Unfortunately it's just something we have to do to continue supporting macOS, and it will push back the Windows version. ARM support for macOS would potentially be helpful toward ARM support for Windows, if there are enough people using Windows on ARM processors to justify doing that.
Mask Image by Brightness is a Luma Key (which, in Red/Green/Blue mode, evaluates those color channels individually) and not a Chroma Key (which evaluates distance from a specified color); it's not great for Chroma Keying. Our community members have had a previous discussion on Chroma Keying.
To elaborate, Mask Image by Brightness measures the brightness based on the Threshold Type you select — if you select Blue, it only measures the Blue channel; parts of the image that have no blue (such as the green background) are made transparent, and parts of the image that do have blue (such as the dog's gray collar) are left opaque. If you select Green, then parts that have green are left opaque.
The plug-in works fine for us. We wonder if it's some type of interference with some of your other 3rd-party plugins. Could you temporarily remove them, and see if the problem resolves? If it is interference, and you can narrow down what plug-in(s) are causing it, we can hopefully figure out why that's happening and fix it.