You always give such insightful answers. I am nowhere as eloquent or deep thinking but I'll give you my sixpence worth.
We all get tired sooner or later and at that time something has got to give.
You did not say after how long this happens.
With very poor technique that could happen after 2 lengths.
With decent technique that could only happen after 50 lengths.
Part of my quest is to swim further faster through better technique and getting fitter.
One could compensate poor technique through getting fitter and grinding away but the poor technique will soon catch up again maybe even with some injury.
TI teaches using the least effort to get the most DPS.
Assuming your starting technique is good, I would say do shorter or slower repeats, building on that each session and stop when things start falling apart. Some video would really help the experts here assist with determining where any inefficiencies could be.