When in XML mode (especially), one goal that's part of XPP is that a round-trip of the data (i.e. an import followed by an export) will preserve the "format" of the original input data as much as possible.

So if the imported data is of the numeric character reference format, for example 中, then XPP "remembers" that and tracks that character in the numeric character reference format so that on export it will be output the same way.

There is no option (to toxsf) to prevent that behavior.

But if by chance you are getting a different character displayed by XPP than the character with the actual Unicode value, then you might need to add the -ncrnomap option on import (for toxsf). That option prevents toxsf from mapping numeric character references via the XCS DEFAULT spec (which can sometimes result in the wrong character in XPP when there are multiple Unicode values mapped to the same entry in the XCS spec).