No. I didn't infer or imply anything. I stated originally that it was customary to use a pitch which would give a stability factor of at least 1.2 with the longest bullet to be used at the desired velocity.
The usual rifling pitch in .375 caliber for the H&H is 12". The old .400/.375 H&H operated in the velocity range you are working with, and I'd be surprised if it had a much slower rifling pitch, or much faster. The closest ballistic counterpart cartridge to yours that I have a good rifling pitch reference for is the 9.3x62, for which the C.I.P. standard pitch is 14.17", and which handles a 286 grain, .366" diameter bullet at around 2360fps. Other cartridges which use similar bullets at even lower velocities may also have SLOWER rifling pitches - the customary pitch for .38-55 rifles is 16", and some of them will stabilize up to 300 grain (cast) bullets to fairly long ranges (at least 500 meters), but others will not. There are too many variables to make blanket statements about what will or will not work (and I didn't), but the evidence you reported indicates that the heavier bullet is not sufficiently stable from your rifle. I will state that a .375" barrel with 12" pitch OUGHT to stabilize any .375" bullet up to 300 grains at the velocity levels you report, and, if it doesn't, there is something else going on.
I've made benchrest and match barrels in various calibers - BR shooters are interested in the very best possible grouping at a given maximum range, and don't care at all what happens just the other side of the paper. They get away with near-minimum pitches because they use the very best, near-perfect bullets they can buy or make.
For .22RF match shooters, I've made barrels with bore and groove dimensions all over the map, from super-tight to fairly loose, as compared to industry standards, and with rifling pitches running from 1-12" to 1-20", according to the desires of the shooter. These folks are absolutely dependent on the quality of the ammunition they can buy, not make, and some have discovered that various ammunitions perform better with non-standard barrel specifications - for them, and under the conditions of their discipline.
I don't like to guess about technical matters which I should be knowledgeable about, and, so far, I haven't - every statement I've made here has been clearly delimited and/or caveated.
Since I didn't make or even examine your rifle or ammunition, I can't know more about it than you do, but I do know about rifles and rifle barrels, and I can confidently say that if I had made your rifle, with a proper barrel, it wouldn't pitch bullets sideways.
'Further deponent sayeth not.'
mhb - Mike