Prior to v2.0.3, Forest would set the baseline node (the node whose anchor will be aligned to baseline) whenever the
baseline key was used, even if it was used on a
phantom node that was not drawn by TikZ. One would expect that this would result in a TikZ error, but it did not.
Once a node was created in TikZ, it's name persists even outside of the group (I guess this could be considered a bug), with the "node" positioned at
(0,0), like here:
\draw[->] (-1,1) -- (foo.center) -- (1,1);
Combine the above with the fact that Forest reuses node names (the root is usually named
node@2 by default) and you get the problem. If you set the baseline node to root (like the linguistics library does) and make root phantom (as is commonly done to typeset side-by-side trees), TikZ effectively sets the baseline to
y=0. The result is shown by OPs example.
Now, Forest 2.0.3 (where trouble introduced by
draw tree method was fixed) got clever and stopped instructing TikZ to set baseline node to phantom nodes (more precisely, any node that is not drawn). Thus, starting with this version, the baseline is not set for side-by-side linguistic trees anymore. The result, as promised by TikZ manual, is that "the lower end of the picture is put on the baseline of the surrounding text", like in the following example. (The nodes are
drawn to better see the alignment.)
This is what we get when compiling the above code with Forest v2.0.3 - v2.1.4.
To be absolutely clear: I consider this the expected behaviour. However, this does not mean it is the desired behaviour ... and this is why the linguistics library of v2.1.5 tries to be smarter: rougly speaking, it sets the baseline to the "first non-phantom node". In our example, that's the left VP, so we get:
(Temporary remark: Forest 2.1.5 is already submitted to CTAN and also available on GitHub.)
Now, admittedly, in OP's example, all this doesn't help at all, as "the first non-phantom node" resolves to
Jane is .... The new baseline default in linguistics works for the common case of side-by-side trees; for anything out of ordinary, simply set
baseline manually, as advised in Alan's answer.