Hey Steven,
Then it must be the either the bit or the spindle slipping downwards. In this case, make always sure to have a clean collet, collet seat, and bit shank. Or the workpiece is lifted by the bit’s operation. Normal bits are upcut which means the bit has the tendency to be drawn into the workpiece. Therefore the workpiece must always be fastened bombproof.
Here a cleaning tool with leather strips for cleaning the collet seat inside taper.
Also is it possible that you made a mistake when mounting the collet?
ER collets are not mounted the way a trim router collet is mounted. Most people put the collet into the axle of the spindle and then they put the collet nut over it. That is definetely wrong! An ER collet has a groove that goes all the way around near the top of the collet. The corresponding collet nut of an ER collet has an excentric extractor ring or offset lip inside. It is absolutely mandatory to fully engage (clip in) the collet inside the nut before you tighten it onto the holder, as shown at this moment of the video “ER Collets | 5 Things to Know for Beginners” . See also this picture from my spindle manual:
– Source: Mechatron HFS Spindle User’s Manual
ER Collets | 5 Things to Know for Beginners - Youtube
ER Collet Essentials. Do You Know? – Haas Automation Tip of the Day - Youtube
The link jumps to the moment in the video where it is shown.