Different query plan used for the same query depending on how parameters are passed

I have a table that includes two text columns t1 and t2, and a composite
index on these columns. When issuing a query of the following form:

SELECT * FROM test WHERE t1 = 'X' and t2 = ANY(ARRAY['Y1', 'Y2', ..])

I have observed that it will use the index and have reasonable performance
if the whole query is passed as a single big string. However if it is
parameterised (I'm using Npgsql) it switches to doing a sequence scan and
performs terribly.

The table contains approx 2.3 million records and the query matches about
20k records (i.e. there are 20k 'Y' values in the array).

I have experimented with changing work_mem, preparing the statement in
advance, ANALYZEing the table, none of these change the behavior.

Why does the query planner choose to ignore the index when the command is
parameterised?

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