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This is because the sender is unaware of your channel balances and how much to split up their payments properly, since MPP is something the sender does.
I'm a bit unsure why the sender wouldn't eventually find the right split, but perhaps there's a time out issue here. And I'm not sure why it's not attempting to create a new channel instead of trying to fit down the existing ones. Perhaps I misread something or I'm not familar with how you are testing this.
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LSPs by definition cannot centralize the lightning network. Centralization is a matter of control. LSPs do not control user funds, the only thing an LSP can do is choose to serve you or not. If they choose not to, just don't use that LSP.
Do not confuse popularity with centralization. They are not the same. A popular service that has no control of your funds or your actions is not a central point of failure even if most people use it. If the most popular LSP in the world begins censoring certain transactions, even that doesn't make it centralized. It is just a signal on the network that there is demand for a new service provider, an uncensored channel to whatever destination the "bad" LSP tried to censor. Anyone can create that connection point easily, so LSPs effectively have no power.