What does pg_stats say about column customer_id? Specifically, how many ndistinct, and what is the sum of the most common values? If you have 1000 distinct customer_id values, and the (default 100) most common values only cover 2% of the total rows, then the optimizer will assume that any given customer_id will yield approx reltuples * .98 / ( 5000 - 100 ) rows. So if your table has 1 million rows, your estimate might be that there should be 200 rows in the table per customer_id in your array.
Looking at your query plan, the optimizer expects rows=8686 for those customer_id and it knows you only want 10 of the most recent ones. It made the right call based on the information it has.
Increase default_statistics_target, at least on that column, and see if you get a much much better plan. I don't know where I got this query from online, but here ya go. I'd be curious how frac_MCV in this changes when default_statistics_target is more like 250 or 500 and the table is analyzed again to reflect that change.
SELECT
( SELECT SUM (x) FROM UNNEST (most_common_freqs) x ) frac_MCV,
tablename,
attname,
inherited,
null_frac,
n_distinct,
array_length(most_common_vals,1) n_mcv,
array_length(histogram_bounds,1) n_hist,
correlation,
*
FROM pg_stats
WHERE
schemaname = 'public'
AND tablename='test_orders'
AND attname='customer_id'
ORDER BY 1;
