There was an assert here as well since using the original object to read
from was having an empty runtime curve cache.
Now use BKE_vfont_to_curve_ex instead of BKE_vfont_to_curve, so we can
read from the evaluated object and write to the original curves in order
to have the modified data taken into account on next object evaluation.
(BKE_vfont_to_curve would read and write to/from the same object)
Final solution provided by @sergey in that report, thx!
fbae4c5ba3