Haven’t seen this before – something of a design problem with our 36V Li battery powered strimmer. It worked very well – sometimes lacking cutting ability on thicker stuff but overall much better than a petrol strimmer. Then it jammed solid.
Dismantling it found this:
The fan at the bottom of the strimmer to pull air through the motor is exposed to grass stems. These can get grabbed and wrapped round the motor shaft with enough force to push the bearing apart.
I did try just replacing the bearings but they are too tight to shift easily. Fortunately a new motor is cheap so I’ll fit that instead. This won’t fix the design problem but at least the strimmer should work for a while.