Of course you really need to look into hitting each version with a big hammer to see if the strength has changed… 
Were all the bolt holes tight when you originally built the machine? Looks like all the cracks are in line with the bolt holes. I’ve not heard anything about pla shrinking over time.
Here’s my working theory… and I have no way of backing it up.
What I think happened is one of the parts cracked from me slamming a bit into something or over-stressing the machine. I can think of more than one occurrence of said case.
Me, in my infinite wisdom, decided that the bolts needed to be tightened because the gantry was now wobbling. Me thinking that a bolt loosened over time, and not thinking about a part being cracked. So I started tightening things. I probably over-tightened something that caused un-due stress on other parts of the machine. The next time I either hit something or tried to cut too fast, it probably caused the other cracks.
I do know I had to use a screwdriver to get two of the bearings out because the part was deformed and had tightened. I think it was the two tension bolt A’s.
The device was also living in my Texas garage where temps would get above 100*F. There’s a chance that the PLA got soft at some point and allowed any bolt tension to slightly deform the part over time.
Like I said, these are the original parts from over 2 years ago.