I had this Vertical / Horizontal issue demonstrated to me on 23cm. Normally I use Vertical because most of my QSOs are to a handi. I tried to work GI6ATZ (mega power Horizontal) from Dent G/LD-045 not a squeek, as soon as I changed to Horizontal 59+++. I had always assumed my poorly constructed antenna would not be that pure in its radiation pattern but not so.
I use a SG-Labs transverter on the back of the 23cm antenna. The driver is normally a 2m Handi.
73 de
Andrew G4VFL