You are getting an overflow. The request
> .sp |(\n[t1]+\n[t2])u
does not do what you think it does.
Section 1.4 "Numerical expressions" of the Troff User's Manual says
In the presence of default scaling, the desired scale indicator
must be attached to *every* number in an expression for which
the desired and default scaling differ.
In your case this means that the formatter is trying to space to
position 383464 _baseline spacings_ (the default scaling for sp)
from the top of the page, which it apparently cannot do, so the
request is ignored.
