That is the normal symptom when this area of the synchro assembly wears or does not engage right: stays in gear under power, pops out when coasting. Why it does not pop out under power, I can't explain, but it does indeed have the exact symptoms you describe. BTW, the 2 cases I have had like this were in 2nd gear one one trannie and 4th gear on another.
Like AJ sez: It won't fix itself. These trannies are not that hard to work on; it is just there are a few tricks to know, and it sounds like this area was missed. The standard rebuild kit these days won't have these springs and keys/struts, so I suspect most people don't know how they work or even what they do!
AJ, I never knew about wear on the clutch teeth so thanks for that.
(BTW, I'll fix it in trade for a 340 block LOL)