I stay working on Sierra (MacOS 10.12.6) and on Blocs I never see any of these problems you people report in forum… 
And the more time passes, the more I become convinced that Mojave and Catalina are OS not worthy of the Apple name.
These are all sites made with Blocs 3.1 to 3.4.x under Sierra OS:
1— https://www.mangla.it
2— Home
3— https://www.prioratodision.net
Sierra is pretty old now and I am not even sure it is receiving security updates these days. Nobody knows yet if it will be supported with Blocs 4.
I have found Blocs 3 very stable under Mojave without the problems being reported by others. On a general level Mojave has been a fantastic OS and my old Mac Pro has never run so well. It seems much better than High Sierra.
Yesterday I installed a new Mac mini running Catalina, so l’ll have to see how that goes.
Hi,
I´m really new, but it seemed I have the same problem!
But for me it seemed to happen only in SM view in edit mode!
VIEWPORT SM -> Change visibility in MD Button. -> NOK
But when I preview it, changes seemed ok…
Is it fixed?
Forgot: Blocs 3.4.9, High Sierra.