It's still unclear exactly why that's the case, but it keeps turning up unlooked-for in different datasets (including from different jurisdictions which use different methodologies)
So far, the most promising theories involve new variants exploiting the immune effects of repeated exposure to the original spike proteins. In particular, the South African and Portuguese datasets on BA.4 and BA.5 lend themselves to this explanation -- but even with older variants, like BA.2/Omicron, the graphs just keep pointing in this direction. It seems that about 4-6ish months after boost, you're worse off than you were pre-boost.
So far, the most promising theories involve new variants exploiting the immune effects of repeated exposure to the original spike proteins. In particular, the South African and Portuguese datasets on BA.4 and BA.5 lend themselves to this explanation -- but even with older variants, like BA.2/Omicron, the graphs just keep pointing in this direction. It seems that about 4-6ish months after boost, you're worse off than you were pre-boost.