I’m noticing this problem across several clipboard managers such as gpaste. That is that when I select something with my mouse, it is automatically copied and becomes the default for my paste. For example, I copy a number with CTRL-V or right click and select copy, and I want to highlight some text to replace with that copied number, the selected text becomes the item that I am now pasting and it shows up as number one in the clipboard for pasting. I then have to manually choose the item I deliberately copied in order to paste, which is now number 2 on the clipboard history.
So the basic problem is that the process of just selecting some text, copies it.
Which flavour of Manjaro are you using, ie. which desktop environment?
I just noticed you tagged as Gnome. Do you have more than one clipboard application running at the same time?