At this moment, libept is not available in trixie (testing):
https://packages.debian.org/search?arch=amd64&searchon=names&keywords=libeptTherefore, synaptic is compiled without EPT support (no "WITH_EPT" defined in compilation).
For this reason, quick search is not available in synaptic.If you download and install libept-dev and libept1.6.0t64 from sid (unstable) and recompile synaptic then quick search will be available.
Best regards.