I'm running KDE on several systems, and have worked in a KDE environment for many years. The version running on my tablet in Antergos (5.13.5-1) is virtually identical to the one running on my desktop in OpenSuse (5.13.4-1.2) but will not allow me to put unique/different widgets and program launchers in different activities. I have 4 activiities, and if I link or add an app icon to the desktop in one from the application launcher, it adds to all 4. The icons now no longer have the pop up handles on hover but a menu on right click, but the whole biggest features I've used KDE for is having different activities. Is this an installation problem, a bug or a new feature?

EDIT: So I tried to solve this by switching in an updated build of SuSE Tumbleweed to replace the Antergos....new Plasma Desktop there is the same, BUT I think I've found a workaround. It looks like if I drag the icon from the Application Launcher to the desktop, then click on the dropdown to select "Add Icon" instead of Copy or Link, it will actually put the application on that activity alone. Some bring Icons with them; some just the name and you have to edit the icon and hunt down in /usr/share for the application folder and find an icon.

EDIT 2: This only partially works. after several reboots and possibly an update those icons are now dead. Interestingly, by shifting to Wayland, the icons work again but any program open on one activity stays open and displayed on all. Still unable to find an answer anywhere on why this changed, and why it affects Plasma5 on one machine and not on my other, which are not both in identical updates of Tumbleweed.

It sounds to me like what's happening is that with new installs, the default desktop type is to show the contents of the ~/Desktop folder. Thus adding icons to the desktop is placing them in this folder which will then be visible in all other locations showing that folder.
When you select "Add Icon" instead of Copy or Link, it creates an Icon widget on the desktop of that specific Activity, while Copy and Link will copy/link the application shortcut file to the ~/Desktop folder.

You can change the desktop type by right-clicking on the desktop, selecting Desktop Settings (or whatever it's called) and then changing the Layout to (I believe it's called) Widget Desktop (or you can review the other layouts and see if any others suit your fancy).