Confirmed - thanks for the report. Another source said that "If a sample number is given without a note, Impulse Tracker will play the old note with the new sample.", but if things go wrong compared to Impulse Tracker, apparently that statement forgets to mention that the new sample must not be the same as previous.
A correction to the occurance in other versions: "Aim for IT playback" was introduced only a few versions ago so this bug shouldn't occur in all previous versions.
Also, if this bug becomes too much of a problem, autodetecting the playmode can be disable by adding lines
[Misc]
AutodetectITplaystyle=0
to ini-file(if there already exists line "[Misc]", the lower line can simply be added below it).