GT Sport wasn't, which is where the numbers came from.
The DR gain in Sport was 80-([Your Start DR-Their Start DR]/500). If the difference in starting DRs was 40,001, you'd notionally lose DR for beating someone. Well, you wouldn't, because decimal places (the outcome is -0.002), but you'd, in theory, show DR down.
But you couldn't. When this happened you'd trigger the exception and it'd be reset to either 0 or 1 (don't remember which).
As they've now obfuscated raw DR data, I have no idea how it works in GT7, but that's how it worked in Sport and I'd be bery surprised if the calculation has changed.
What has definitely changed is the DR bar display bug, and I'd be unsurprised if this bled into up/down arrows. Plus there's the whole quit-lag thing noted above.
Absolutely, if the person you beat is very high-ranked, and the people who beat you don't take too many points (by again being a lot higher-ranked).