@Etonkovidova and I have noticed this issue in production for the mobile homepage. This screenshot is from Czech Wikipedia in Safari 12:
@Catrope is working on this now.
Change 540959 had a related patch set uploaded (by Catrope; owner: Catrope):
[mediawiki/extensions/GrowthExperiments@master] Homepage: Don't use flexbox for vertical layouts in mobile start module