If the app shows clearly what needs to be done (shop, order list, miles driven), and the pay the worker will earn, and asks if they want to accept, then IMO that's fine.
The business can set those offers however they like, even using a random number generator if they want, and IMO it's morally fine.
If the algorithm detects that you are likely to accept for little money and short you with lower offers compared to other users, is it still morally fine?
Consider what risk might exist if you fear overpaying so much that you make a lowball offer yet someone feels compelled to accept. The product or service might be "done" but in a way that screws you over in the long run as well.
Trust is earned, and it flows both ways.
Ideally, there's incentive for people to collectively reach the most efficient solution through aggregated laziness and greed.
In practice, people only have so much bandwidth and shortcuts will be taken, options will be overlooked, and people will exploit or be exploited due to the blinders either put on willingly or forced on them--on top of our natural capacity for observing reality no matter how much information is provided.
Anyway what is moral depends on your personal values, not the law nor how things are done, it's expected to disagree.
Platforms sometimes care—by which I mean, achieve a market position that means sellers can’t afford not to use them, then leverage that power to force lots and lots of weaker people and entities to do what they want, possibly causing higher lowest-prices in the overall market in the process, so, also hurting buyers.
At least with laws, there are clear adjudicators on the issues at hand.
When it becomes immoral from your perspective?
Or if they get you to pay them money upfront (ie. for uniforms) on the basis of 'workers earn $Y per day', but then change the rules so some workers don't earn Y per day and don't offer a refund of the upfront payment to unhappy workers.
Time-based contracts are pretty normal. I imagine most people on the planet are on them. There are exceptions - e.g. sales commissions - but to say that workers lose on the thing that most people do requires at least some elaboration.