using D21 on a Pi4 do not work correctly

We use a small strip of 40 leds RGB, we get several trouble wit colors specially with high brightness. We found a workaround using D18 pin that work perfectly. Analysing the signal there is a strange delay on the D21 pin:


there is 11 bits and then a small waiting time that as no sense

using D18 the whole bits are transmit in one shot :


Start by reproducing issue #160 on a Raspberry Pi 4 with the 40-LED RGB strip, comparing D21 and the reported working D18 setup at high brightness. Inspect the captured signal timing and define done as consistent colors without the reported delay-related problems on D21.

Written by the indexing model from the issue text.