Was their any evidence of potential serious malfunctions arising from that original FADEC software?
In short, yes. However, one could argue the main problem was lack of understanding of how FADEC was meant to work. Maturity of understanding leads to a safer system. Maturity is key to this whole issue. Under the mandated rules governing System Maturity, there was no authority to enter Production. It could be argued there was none to enter Full Development either, although I'd argue that would be too strict an interpretation.
In 1993 MoD sent a team to Boeing to learn as much as they could about FADEC, so they could "train the trainers". A misunderstanding meant they were greeted by Boeing staff who thought MoD were there to teach THEM!! Again, a former RAF officer who led this team came forward during the Philip Review. This tells you much about why FADEC was such a mess. Boeing had little to do with it due to the contracting arrangement. They were expected to simply sit back and wait for MoD to deliver it. Compounded by the LRU hosting the Safety Critical Software (DECU) being managed by a mechanical committee, not the correct electronic committee, which would never, in a million years, have permitted such a nonsense. Again, the evidence to Lord Philip explains this in detail, with a quote from the deputy chair of that committee.