Qt5 packages had separate GLES versions packaged in Debian, but on Qt6
the rendering backend was revamped by Qt so that the same binaries can
support desktop GL and GLES.
For some reason, the current Debian packaging does not include libGLES
development headers in Build-Depends. When configuring the build, Qt
will not be able to enable GLESv2 support. These packages then cannot be
used to render anything on devices that do not support desktop GL, but
have GLES support.
I have confirmed that building with GLES packages indeed fixes the errors.