Does not seem strange to me... some cells are just not able to deal with higher discharge rates. Many laptop cells are like this, and if this is the case, you have to be extra careful when building for something like an ebike. In high drain applications IR measurements are a must.
It doesn't matter if you are doing a big or small project. If you want to do it right, you have to make the investment into the tools.
Also, make sure your testing procedure is not flawed. If you are using an RC charger for testing at higher currents, then using a 4 wire measurements (with balance leads) will provide you much better results.
You should also look at the datasheet for the cells to make sure you are not applying them incorrectly. How many cells in parallel are you going to have in the ebike pack and what are you max loads?