There's something about the design of the bottom (probably that it flares out quite steeply) that makes the vase mode prints unusually weak. My print had the bottom split from the top just as I removed the plate from the magnetic bed.
This has never happened with any other objects I've printed in vase mode.
To be fair, this was printed in PLA silk which is more brittle than standard PLA.
You are probably correct that the connection between the bottom layer and outer wall is made weaker by the taper. I will remember that for future designs. Thanks for the feedback
Vase mode prints are only connected to the bottom layers by a single line width. It doesn't matter what model you print, it's going to have a weak connection to the bottom layers. Rather than trying to pull it off the plate with force, use a spatula or something you can work underneath it to pry it off the built plate rather than pulling it. This is applicable for any model that you print in spiral vase mode.