I've been able to reproduce the issue using the standard map with value heatmap example, especially if the data set is left un-truncated. My feeling is that this is a bug within the canvas rendering, as I have come across a similar issue before (but with overlays). A workaround suggested there seems to work for me here as well. I never get any overlay rendering problems when using DOM mode - it seem to get this problem only seems to occur in Canvas mode.
To force the library to use the DOM mode, change the <script> tag as shown.
Meanwhile I'll ensure that this gets raised as a bug internally, so hopefully a fix will be in place in a subsequent release.