Depending on the converter.. My Progressive Dynamics Wizard controlled unit every so often does a controlled "overcharge" and that looks like what you are seeing. This serves several functions.. It stirs things up a bit so you do not end up with very weak acid at the top and very strong at the bottom (Stratification is defeated) knocks a bit of sulfate off the plates (Desulfication) and equalizes charge across the cells.
However if it continues for long long periods of time. You have a problem. Or may have. I understand there is at least one brand that calls for that kind of voltage when charging... NO, sorry do not recall which.