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After looking at DWDS I would agree, that this is a dated combination and no longer used with notable frequency.
Checking your source I have to mention that your first finding combines two actually two very different constructs:
liegen lassen is a fixed phrase of not taking care of an issue
die vor uns liegen, zu unterschätzen: here the correctly identified infinitive is in a separate clause and unrelated to liegen; the relevant phrase is vor uns liegen ("Don't underestimate problem, which we expect to become pressing soon"), so vor can be considered a typical preposition indicating local/time relation. Similar prepositions in frequent use are auf, hinter, unter.