If it feel like it’s doing the wrong thing, it probably is. I get weird text-selection behavior when I start running low on memory. In general, though, any formatting like that I try to do within a paragraph in AE I end up regretting once I start to animate. Invariably, I end up deciding to animate separately that header line that was 10 points larger than the copy, or the mid-sentence bold phrase. Now, I usually just give them their own text block so I have flexibility.
Also remember to note that although AE doesn’t acknowledge it very much in the interface, there are two types of text: Text and a Text block. If you just start typing, you’ll just get text. If you use the text tool and drag a box before you begin typing, you’ll get a text bounding block that you can insert text into, which will also create the line breaks automatically based on it’s size. You may be noticing slight differences between them. Resizing basic Text results in the text changing size (squash, stretch). Resizing the Text Block will re-flow the text.