OK Gary,
think I found it!
After some experimentation I could also reproduce the behaviour you described.
I found that the error occurs when the QSK DELAY is less than the length of a CW element.
In your case you experimented with 16 WPM, the length of an element is then 1200/16=75 ms.
So at 16 WPM, when the QSK DELAY is less then 75 ms the problem will occur. This corresponds to what you reported.
If you use a slower keyer speed the problem will already occur with longer QSK DELAY times.
Now that I know the root cause it's easy to fix it in the sketch. I will release v1.27.2 with a bug fix shortly.
Please try it out and let us know if it all works OK now.
Again many thanks for your detailed reports and thorough testing!
73 Allard PE1NWL
Allard PE1NWL