Hi Clint and Welcome,
The onscreen keyboard behaves as follows. On a 2 button mouse for a right hander the keft button is the momentary note clicker. It does not lock notes. When clicking the keys with this button the keyboard is velocity sensitive so it matters where you click on the key as to how loud it will make the note be because of the velocity value. This is especially noticable on Velocity type instruments like pianos . This pic shows the velocity range for a click on a single key from hardest velocity to softest velocity .
It’ might explain why it got loud on that click you mentioned.
The right mouse button locks the note you click it on and it stays on till you click it again or hit the panic button on the upper right of the toolbar. You don’t need Ctrl keys for it to work. It sounds like you have discovered this behavior already
Hope this helps and