So it does not seem to be caused by an external sticky header plugin.
I already tried:
restoring previous header versions;
clearing browser cache;
adding CSS fixes for .o-sticky-pos-top and .o-sticky-bnr-keep;
checking active plugins.
The issue remains.
Could you please tell me where this sticky behavior is controlled in the Extendable theme, or provide the correct CSS to disable/fix it without breaking spacing and layout?