Textblocks are really supposed to be used in vertical mode, and a textblock forces a new paragraph if it starts in horizontal mode, so this is to some extent unsupported territory. The TeXnical reason (thinking about this briefly) is that the
textblock results in the ‘Line 2’ becoming separated, within the paragraph, from the ‘Line 1’ above it, so that TeX ‘forgets’ that it's in the middle of a paragraph, and so that has to add inter-paragraph spacing (ie, there isn't an upward vertical shift here, as much as a lack of the downward inter-paragraph space which TeX would otherwise add).
However it wouldn’t be unreasonable in this situation, nor necessarily particularly hard, for textpos instead to adjust spacing and penalties so that a textblock in horizontal mode ends up acting more like a
\par (that would be me, editing it). That's not going to happen immediately, though.
The most robust solution in this case, though, and with the current version of textpos, would be to move the textblock, somehow/somewhere, so that it ends up between paragraphs. It'll be happier there! In the beamer context (though I don't have much experience with beamer), it might be that putting the textblock (or textblocks?) all together at the beginning of the frame, might be worth a look.
I've added an issue in the repo. Thanks for reporting this.
Edit: clarified 'wouldn't be unreasonable' remark.