Perhaps it's starter is in fact cranking in the proper direction BUT, when it fires, it reverses!!
Story time!!!
I recall in my model airplane days, the "cox .049" engines etc (although reedless 2 strokes) that you'd have to hand/finger start (unless you have once of those fancy manual electric rotation thingies that spun it for you) could sometimes jump into running backwards although you were spinning them forwards when starting them.
I recall this quite vividly because when it happened, I was like WTH and my dad said it happened sometimes and just throw a rag into the prop to stop it and try again........
End Story...
Back to the merc and timing, I'd think before TDC should become after TDC if it used a trigger setup but it would still run, poorly, but it'd still run. Now, I'f it were points however, I'd think it'd run quite happy in reverse as points would run symmetrically either way.
I guess IF it's actually doing this, runnign backwards after the starter was turing forwards that is, I'd be asking why now? Perhaps a particularly specific type of failure or wear or mouse in the engine or something has occurred and is allowing this ...IDK. If my motor did this, I'd be a bit perplexed but since my airplane's ran backwards, I wouldn't claim Aliens are involved......not yet anyway.
Just my .02