From bug 161049 comment 8: If no "latest tab" is yet remembered, then users have to select a specific tab instead of having "Numbers" pre-selected already. You can see this by running in Safe Mode. The dialog opens blank, which is jarring.
This has started with https://git.libreoffice.org/core/commit/2199e603691a770ea6b67cbaba4ce1e0ce7b1919 tdf#99528 Use vertical tabs in 'Format cells' dialog Visible in bibisect repos with rm -rf instdir/user/ ; SAL_USE_VCLPLUGIN=qt5 instdir/program/scalc