Advertising, or trying to redirect to other restaurants paying higher commissions are the only possible explanation (aside from incompetence of course, but if you believe that I've got a bridge to sell you).
They're trying to maximize revenue by double-dipping. They want the $70 food order and another $X because you viewed some ads.
- You search for a specific BBQ restaurant on Google.
- You see an advertisement for that restaurant! But it's actually DoorDash.
- You click on that ad, and DoorDash initiates a location-based search for BBQ restaurants, putting their preferred partners at the top of the result list.
DoorDash is happy if you order from any restaurant through them, but they're even happier if you order from a restaurant that they get more money from.
Is that that what they prioritize over having a functional app for customers, merchants and drivers alike, and proper development deployment that doesn't incur large blackout periods?
Doesn’t Google let anyone bid on any word, including brands? [1]
Doesn’t it stand to reason that it’s at least plausible that other search services might do the same?
1. https://www.google.com.au/search?q=google+lets+anyone+bid+on...
DoorDash and Grubhub make the overwhelming bulk of their revenue from simple fees on the deliveries made. They surely sell ads (and para-ads like list placement), but the eyeball value of my one visit is vanishingly small relative to the $70 I just spent on takeout. The scales are just totally different.
If they're not showing you the restaurant you followed a link for, it's not because of advertising. It's likely because they don't have a contract with that restaurant and still want to sell you a meal.
Now, that's not ethical. But it's not because of "advertising".