Hi and welcome here!
If you are using the VST3 of Sampletank, than this a known problem with IK Multimedia VST3. This happens with other VST3 like Miroslav Philharmonik2 too, also in other DAWs like Samplitude and Cubase. What I know, this occurs with hanging notes, but maybe this affects the patterns too.
Try to use the "normal" VST instead.
You can get more information about that in this thread:
"Stuck" notes with Miroslav Philharmonik 2
Hope that helps,
Peter