We will begin with the following map, which illustrates the current routings of US highways 136 and 150 between Indianapolis and Bloomington. From its eastern terminus in Indianapolis, US 136 (the blue line) heads west to Danville. US 150 (the red line) approaches Danville from the south. The two routes "bump" there before separating again, and then they have another intersection just south of the town of LeRoy IL:
If you zoom in a bit, you can see that I-74 between Indianapolis and Danville was built along the same corridor as US 136. Then between Danville and Bloomington, I-74 separates from US 136 and instead follows the US 150 corridor.
Why did the two states want to make this change? During this timeframe it was not uncommon for DOTs to eliminate older US routes whose functionality had been replaced by newer, parallel interstates (thereby eliminating the expanse of maintaining the redundant road). So that explains why US 136 might be deleted between Indianapolis and Danville. But west of Danville, US 136 served a different corridor than I-74. So if they wanted to truncate US 136 all the way to LeRoy, then what did they intend to do with the segment of US 136 between LeRoy and Danville? This next map illustrates what I believe the two states were planning to do:
So basically this action would have eliminated the redundant US route all the way from Indianapolis to LeRoy. Sounds reasonable, but then the question becomes: why did AASHTO approve the proposal only between Indianapolis and the junction with US 41? Here is what I think: the proposal explained only the changes to US 136, but Illinois did not specify the related changes they planned to make to US 150. So AASHTO deferred taking action in Illinois, presumably requesting more information from them.
One would expect to see a revised proposal submitted for consideration at a future AASHTO meeting. But none of the meeting minutes in subsequent years reflect that, so it would seem Illinois must have backed out of its agreement with Indiana. And it appears Indiana changed their mind too, because as far as I know they never truncated US 136 to its junction with US 41; the route is still signposted all the way to Indianapolis.
But it seems Indiana never went back to AASHTO and told them they had reversed course, because for years (at least until 1989, and maybe to this day) AASHTO's route log listed "Veedersburg" as the eastern terminus of US 136. And where is Veedersburg? You guessed it: at the junction of US 136 and US 41.