I believe (this isn’t backed by any data) that the majority of people who buy marketplace products are those who also update their projects to the latest version, nag you about making an old product compatible for the latest version by posting meaningless questions, and just in general can’t develop much of anything themselves.
As such, if the latest version doesn’t work or you don’t have the thing you released compatible with the latest version, then your sales are bound to drop any time a new version is released.
Ofc if you make things without code that are always compatible with the next engine version anyway, then there is no explanation for a drop in sales if it isnt backed up by lower reviews or similar external factors.
Either way its not like the marketplace provides us detailed analytics to understand what is going on, so its anyone’s guess…