I wouldn't worry too much about it. Mine did the same thing last year and it did not affect the plants growth or yield.

Just as with the fruit rapid growth accompanied by inconsistent soil moisture levels - too wet and then too dry etc. can cause stem splitting. The stem shrinks then swells and splits. Stabilize the moisture level.

That plant is going to be awfully big by the time you can transplant it outdoors. Personally I plant some new seeds for proper planting time or at least take small cuttings from that one and root them.

Dave