Why do some WS2813 Ledstrips shift/randomize colors?
The Teensy revisions 3+ use 3.3V internally.  The pinouts will provide no more than this voltage.  The LED, on the other hand, requires 0.7 x 5V = 3.5V as high input.
You need a voltage translator or buffer chip.  Any 74ACT or 74HCT logic chip will do.  It also helps to connect a 100 Ohm resistor between buffer output and LED data input.  See https://www.pjrc.com/store/octo28_adaptor.html for related information.