Unfortunately, there's not enough in-universe clues to sync the shows up, but most likely the answer is that Flash and Arrow are out of sync. There's a slim chance a future Flash episode will refer to this event, and we'll know for sure, but probably not.
This isn't unusual, and in fact, the exact same thing happened last season. At some point, Flash ended up out-of-sync with Arrow, and Felicity and Ray appeared as a couple on Flash, trying to figure out the ATOM suit, at a point where that made no sense on Arrow. Similarly, it's very difficult to fit the two season finale crossovers (specifically, Oliver being in Central City, in League armor) into their respective timelines, and no hint was given how that was possible.
The most reasonable explanation is that Arrow is lagging behind Flash, and this funeral and related events took place before Barry lost his speed.