This is embarrassing, but its such a simple issue, I don't even know where to start to fix.
Just recently this started happening in the last few days.
When i start a simple formula - for example =10*B10 - the B10 remains highlighted so then when I type another function or click on another cell, the new cell or function overwrites the cell.
To write the formula I have to move the cursor up to the formula bar and click to the right of the highlighted cell to unhighlight it and then type the next function or number.
This is reminiscent of decades ago when you had to be careful about the 'insert button' or you would overwrite your text instead of inserting.
But toggling the insert on and off does not fix it. In fact I am not even sure the "insert" button is real any more.
Yes its maddening and makes the program worthless.
Btw, I am on Office 365 MAC OS Mojave. Mojave is the only thing that might have changed recently.