Third gear popping out after linkage repair

So on my MK8 Manual Fiesta my gearbox started to experience an issue where I could not move the stick to the right of the box to enter gears: 5th, 6th, and reverse. After fiddling with the adjustment on the linkage it was determined it was not the problem. Turns out there’s a square plastic cube which sits on top of a ball joint which was missing, after replacing the square it started to work again. 
 

Now the issue is, is the gear box has gotten stiffer, presumably due to a brand new part being placed in it. Now occasionally my third gear will snap out of place and go back to neutral unless I really push it forward (this has never been an issue before).

We’ve tried re adjusting the gear linkage by putting the car into fourth gear and realigning the linkage to match 4th, but still the issue persists. 
 

One theory we have is that we only replaced one of the plastic squares, not both which are present which may be causing some sort of misalignment? Another theory is that I’m just not used to the stiffness of the gear box now so maybe it’s my driving? (Although I’ve had the car since 3k miles and never had an issue with third gear). Our final theory is that the part simply needs to be worn in with some driving for it to sort of go back to normal. 
 

Note: it’s a very rare occurrence for it to snap out and it only happens when I’m on a bend which tells me it’s my driving, but previous to this issue I never had this gear problem before, which tells me the distance to move gear forward to get it into third has increased.

This only happens when the engine is running and revs are applied, when the car is off the gear doesn’t snap back at all. 

Any thoughts on how to solve this issue?

Thanks.