Hello once again.
It seems the reason why this was happening is because I had enabled "Script: ItemTakeVolume_via_Mousewheel.eel" action and assigned it to mousewheel(obviously
).
Although I had enabled the "Items full" option within the lock settings, the aforementioned action somehow overrode the lock settings and allowed me to change take volume. I have a feeling this is a glitch which perhaps needs some fixing.
As a way to work around it, I combined the mousewheel with another key (in this case, ctrl&shift) so that I will not accidentally change take volume by simply hovering over an item.
This way, when I have the "Items full" enabled in the lock settings, the volume will not change as intended.
I believe the script mentioned above is a third party one (I could be wrong!) and it somehow defies what Reaper tells it to do. (I have no knowledge of programming nor scripting whatsoever and don't have a clue as to why this is possible, but it was happening nevertheless
)
Anyway, here is a temporary solution for those who could be facing the same problem☆