Block not visible (or false warnings) when trying to drop open another screen block

When I start a brand new project, and try to drag and drop an "open another screen screen name block" (from control drawer) it seems to fail.
Then (when I finally suceed after several trials) several false warnings appear.
By the position the screen centers when selecting those warnings, it looks like several instances of the another screen block are there, but they are invisible.
Does someone else noticed this, or do I have a problem with my mouse?

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Possibly this issue ?

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Chris Wards explains well (with different block)

Closing AppInventor and loggin in again, makes all the missing blocks appear .
(you might think only one appears, because all of them are exactly in the same position, one under the other)
Of course, Warnings mean they are not connected to other blocks.
I use Firefox as well