LO,
The F135 is established and mature, with production engines set for delivery later this year after more than 12,000 hours of testing.
It's what happens when bragging about the new product (drinking one's own bathwater) before the product is completely wrung out, i.e., flight tested throughout the flight envelope. There are differences between the F119 engine and the F135 engine and therein lies the problem. The F119 has only a one stage LPT while the F135 has a two stage LPT, weight balance change, an added 17 inches in length plus higher temperatures to achieve higher thrust to weight capability. As the old saying goes, "Don't count your chickens before the eggs hatch".
The longer-term concern is whether the root cause (the fact that flexing of the engine wasn't accurately modeled or found in flight-sciences testing) will bite us later.
IMHO, this is the real problem. Also, as I recall, the seal in question isn't a seal between the tip of the blades and the outer engine casing, but one to the inner compartment surrounding the LP shaft.