In then middle of a fairly simple wood block passage there is an empty bar into which I have been trying to input a quintuplet of eighths. Every time I start inputting, it shifts all the following notes two the right one at a time for every one of the quintuplet notes I enter. I’m using 3.5.12, and thus far I’ve not run into this problem when going back and putting notes in an empty measure.
I guess you inadvertently have switched on Insert Mode (shortcut I). You can see it on the left side of the screen as the I-beam icon:
Simply type the letter I again (or click the icon on the left) and you’ll see Insert mode is off now.
Hope this helps, and BTW, welcome to the forum!