When one who has rendered prapatti (a prapanna) commits sinful acts such as stealing, cheating, treachery, etc., will that person be affected by the aftereffects of those sinful acts? Will that person’s sharanagati fructify?

There will certainly be grievous consequences for such a person. Regardless of whether one is a prapanna, one who is devoted to Perumal, or one who renders kainkaryam-s for Perumal, one will be punished for their transgressions. No one can escape the consequences. Deliberate wrongdoings will inevitably result in punishments and such people will face severe impact.
The sharanagati performed by such people will fructify; like how sinful acts lead to punishments and good deeds yield favorable results, sharanagati which was performed for salvation (Moksham), will lead one Moksham.
These sins will not act as obstacles for Moksham because such persons will undergo the necessary punishment on this Earth during their lifetime. After having completely experienced the aftereffects of the sinful deeds, before attaining Moksham.