From Beta 2 release notes:
"Multiple usability improvements to source timeline viewer workflows."
I really had hopes that this would include improvements to the Source Tape/Source Clip modes, but I just installed Beta 2 and unfortunately, this problem still exists.

To make the fix simple, can we please have the option to change the default source mode?
- Right-Click the Source Clip or Source Tape button and then "
Set Source Default"
- An option in the three dot menu of the Video Preview panel.
Either one would be great. I'd rather go back to Source Clip view as the default source mode.
EDIT:Actually, I found an even easier fix. There are a couple settings for toggling the source/timeline views:
View > Source/Timeline Viewer
View > Source Clip/Source Tape
The first one switches between Source and the Timeline view
The second one switches the Source between Source Clip and Source Tape
When you use the second option to select which source mode to use, the first option does respect your choice and remember the one you selected when toggling between Source and Timeline.
The Source button on the Speed Editor should do what the first option (Source/Timeline Viewer) does and remember whichever option is selected by the Source Clip/Source Tape option. But the Speed Editor does not remember the selection, it always goes to Source Tape and there is no way to change this.
For now I at least have a decent work around. I've always used the Speed Editor for this and haven't ever used the menus or keyboard shortcuts, but this will work for now until it the Speed Editor is fixed.