I wrote a post about this yesterday as well, but deleted it as I thought I had found a temporary solution, however I'm still struggling with this.
In my mind the vertical alignment (valign) settings of text inside a fixed size text box should follow that specific text box and not the text style, but I noticed that the text style is set to overridden when valign is changed. When I became of aware of that I thought that making a separate text style with the valign set to "bottom" it would work as expected.
But it seems that the valign setting on a text style is removed when syncing with or drag/drop from DSM.
I made some screenshots for you to see what I mean. The red frame marks the text frame.
The text styles are exactly the same, the only difference is that the one on the left is connected to the text style, while the one on the right is detached.
I tried to copy/paste the symbols to an empty document and the valign setting was kept, so this seems to be an issue with DSM, and not necessarily Sketch.