No Idea how it happens, nor how to reproduce:
but it seams to be possilbe (Mac, Version 7 (7.11.21271.01002, 2021-09-28) that the sweep1 dialog starts with no Frame Style and no Preview.
as soon as i klick on one of the Frame-style options i get a result.
but the command should start with a default frame style, or the last frame style
run sweep1 on the left setting.
green surface edge as rail.
framestyle align with surface
create the sweep to store this setting.
now run sweep1 again on the right setting, curves only.
align with surfaces is now greyed out - which makes sense
but the framestyle should fall back to freeform. (expected behaviour)
what i did not check - if this also happens, when the first sweep1 was done in another document that is opened at the same session / in parallel.