Tentative guess: this might actually be expected due to baseline-alignment being different between vertical vs. horizontal writing-modes. The alignment on the central baseline might end up having the text be aligned at a 0.5px-different position in a vertical WM, which then ends up making the line 1px taller. Similar to what was going on in bug 1808788, where we also ended up with an explainable 1px difference in a vertical writing mode.
Bug 1823516 Comment 1 Edit History
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Tentative guess: this might actually be expected due to baseline-alignment being different between vertical vs. horizontal writing-modes. The alignment on the central baseline might end up having the text be aligned at a 0.5px-different position in a vertical WM, which then ends up making the line 1px taller. Similar to what was going on in bug 1808997, where we also ended up with an explainable 1px difference in a vertical writing mode.