Works as designed. The binary undroidwishes are built from the
wtf8-experiment branch which uses a variant of TCL_UTF_MAX=3
representing unicode code points beyond 0xFFFF as a surrogate
pair internally. This is documented in the short release notes
in http://www.androwish.org/download/index.html since the
release "The Leyden Jar" (2017-10-11).

If you want the same behavior as in AndroWish you need to
compile your own undroidwish with TCL_UTF_MAX=6 by changing
the relevant build-undroidwish-*.sh script.