Hey! Thanks for the warning; we already encountered a similar problem with polygons, which I made an issue for on the PsychoJS GitHub. I added a link to your post to say it’s probably also happening for text stimuli: Use variable in the size of VisualStim -> First frame it's default size · Issue #293 · psychopy/psychojs · GitHub
For a workaround, see: Experiment uploaded to Pavlovia has brief black spots and after-images from previous trials - #3 by DavidBrocker (in your case, set the text property of a textbox to some value, like " ", in the Begin Routine tab of a code component).