In general all the cyrillic symbols look better than 1.13-pre7 but some of them still look odd especially the lowercase "ш".
Commands to reproduce this:
/tellraw @p {"text":"АаБбВвГгДдЕеЖжЗзИиЙйКкЛлМмНнОоПпРрСсТтУуФфХхЦцШшЩщЪъьЮюЯяѝ","bold":true}
/tellraw @p {"text":"Ж ж Ф ф Ш ш Щ щ Ю ю","bold":true}
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Please check if that's still an issue for you in the latest 1.15.1 stable release or the latest 1.15.2 (pre-)release.