Leading Not Working Properly

What I am trying to do is very, very simple, which makes this all the more vexing.

I simply want to make a text box containing the numbers "2023". I want to drop the "23" down a single line, so that it appears directly beneath the "20" without having to create a completely new text box. Normally, I would just place my cursor between the 20 and the 23, hit enter once, center justify everything and be done with it. This is how it works in Photoshop, Illustrator, and literally every other program with a text editor I have ever used in my entire life. For some reason though, it appears to work entirely differently in InDesign.

When I try to do this in InDesign, and I hit enter once to drop the "23" down one line, the 23 just disappears out of view, and I get the little red "+" icon noting that the rest of the text has gone outside the bounds of the text box. This shouldn't be happening though, it's quite a large text box with only 4 numbers typed in it and no crazy leading values applied. Changing the leading value actually does absolutely nothing, I can set the leading down to the bare minumum of 6 and it still will not bring the "23" back into the bounds of the text box. I looked up the issue and have tried every fix I could find--  I can confirm it is not set to align to the baseline grid, and I do have the option checked to apply leading to the entire paragraph. 

Any help would be immensely appreciated.