I'm kinda pasting here from my previous comments from when this has been asked.

There was a traditional cosmology that could be interpreted as such, but it fell out of favour after the Jesuits became more established in the court. However just that it existed isn't to say that it was anything more symbolic. However that may be what OP is thinking with this question.

There were also astrologers, both around the beginning of the common era (a.k.a. "the time of Christ") such as the Han era scientist Jīng Fáng, and a thousand years later — Shěn Kuò of the Sòng period — who argued for celestial bodies being spherical, but I'm not aware of them ever stating that the Earth itself was as well.

The thing that's important to remember is that this is a cosmological worldview. It stuck for so long in the same way that there's all the scientific evidence in the world for evolution but yet there are still people in powerful positions that are Young Earth Creationists. It's a fundamentally religious type of mindset, so the argument that ships masts are the last to disappear over the horizon isn't sufficient reason to doubt the veracity of the claims of a flat earth before the Jesuits' approach took hold in the court.

Did this cause problems for navigation? Did Zheng He also believe in flat Earth?

Zhèng Hé was operating on preexisting trade routes following prevailing winds and coastlines. The north star (as an example) could have be observed as being due north for millennia so there's no reason to know the shape of the earth to know that that's north when it's already a given that it's north. Navigation was thus not particularly an issue.

As for whether Zhèng personally believed the earth was flat, I don't believe we have any record to say one way or another. We know the astrological/cosmological/scientific texts of the time had an opinion though, so we might assume his view was consistent with that. But that's speculative because in reality we just don't know (unless someone has seen a diary of his that I haven't heard of).