Split/Aranged/Tiled Spread does not keep 50/50% size while working in one of the windows

Hi

I am working in Adobe InDesign 2024 on an IMac 27".

When working on a 2-up, 3-up or any arranged/split windows the windows keep on resizing with each action I take so that a 50/50% 2-up split ends up more like a 70/30% split. 

Is there a way to lock the arrangement so that it does not resize by itself?

The attached screengrab started as identical pages of 300x250 pixels on a 50/50% split but while working on them you can see the window on the right side shrank in size while the window on the one on the left side grew proportionally.

Is it a possible bug?