It's not exactly what you want - more a work-around [Windows 10]:
Don't save the selection as an alpha channel but as a layer mask and deactivate the layermask.
I think you know that in this case the image's background layer has to be changed to a normal layer.
For editing you can activate the selection without the masking effect in the channels panel.
If you want to transfer the tiffs to another program for further processing: I don't know if this kind of selection / channel is accepted.
Thanks, I'll try that at home tonight.
But with alpha channels, it is normal for Windows to show the TIF thumbnail in that "broken" way of showing only the first saved selection? I don't know why Windows would want to do that. If it's normal, it seems like something that needs to be changed in later updates of Windows.
I see how that works now. Thanks for your advice.