Just a follow up on this. This bug is now fixed in version 1.8.8 of PixInsight, which I'll release very soon. As an added benefit, the real-time preview window is now faster and more responsive to tool parameter changes. There are other planned improvements to this interface that I'm considering to implement also in the new version, not that I have opened this 'can of worms'
Thank you again for your help and support.