IBEX is a small satellite, with less than 1 square meter of area visible. In addition it is very far out, at the closest point is is 59,000 km or so away from Earth. Space Track has stated that they can track an object at Geosynchronous orbit that is about the side of IBEX. It seems that it can only be tracked when it is at it's closest, and there are definitely some holes in that tracking even at the closest approaches.
That being said, it is unusual to have no TLE updates for such a long time. From the very data you showed, however, this isn't the first time that such a long gap has occurred. It happened between 2012-2013.
If I had to speculate, I would guess that JSPOC isn't tracking these objects are carefully for some reason, which could be that the closest approach is happening over the Pacific ocean or some other very remote location, and it isn't able to be tracked when it is at that closest time period.
NASA not releasing status of a spacecraft that has been dead for 4 months would be unprecedented, and I don't see any reason why they would hold back. The mission is well beyond it's planned lifetime. In fact, IBEX has released data as recently as 2019, so while there seems to be some difficulty tracking it from JSPOC, it is there and operating.