The revised flood selection tool in v2 seems very temperamental, and very often does not make any selection at all when working at pixel level. Suspect there's some weird coordinate rounding issue happening behind the scenes? See attached video where I try to make a 0% tolerance contiguous selection on single pixels ... most of the time, no selection at all is made (unless I luck out and hit the zoomed-in pixel just at the right place for it to register).
affinity-photo-magic-wand-flood-select.mp4