was the programmed feedrate in inches per minute or inches per revolution? I can understand i.p.m. messing up the threads if the rpms change, but you'd think that i.p.r. would sync to the spindle speed.
It should be possible to cut tapered threads in G96 (constant surface speed) mode if the feedrate is programmed in i.p.r.
Also, (If I recall correctly), the Cincinnati lathe I ran years ago locked out any speed or feed overrides in threading mode.
Even the vertical machining centers lock out speed & feed overrides while in tapping cycles.