I take these things very seriously, I have been wrong on a part and found out about it in the forums.
I spent the last 40 minutes double checking all the middle parts. The ones on thingiverse are correct. The dimensions you gave do not match either the international or standard piece. I do truly appreciate your looking into this. Could I ask you to double check that with a fresh download. I could still be wrong but I regenerated a middle joiner and compared it to the one online and they are identical. The number you should be getting is 24.8mm. Taking dimensions off STL’s is not very precise but your number was so far off it made me go back and check. I’m not sure how you are locating the center of a “hole” on an STL since they aren’t round, I suspect that is the problem. Meaning the bolt holes for the bearings.
I would prefer a private message to take care of things like this to make sure misinformation is not being spread needlessly. I have no problem being wrong publicly, but in case I am not wrong I do not want to discourage people from building this because of a post like this.