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This turns out to be a Weaver Xtreme issue involving the user setting a higher base font size (22 instead of 16). The calculations to set the size for the Block Editor were not generating the right CSS. Seems to look good now, although the font size stuff in the Classic editor may not reflect the exact size when the base font size is changed – will just one size.
This fix will be in Weaver Xtreme V5 which I plan to release on December 26 or 27.