Problem is fixed when the where part of the cached query is removed. I'm not sure the following configuration is ideal but what I understand is the aim is reducing the count of queries.
<entity name="product" query="SELECT product_id FROM Product WHERE 1">
<entity name="strength" query="SELECT *
FROM Strength WHERE product_id = '${product.product_id}'">
<entity name="form" query="SELECT CONCAT(parent_route,'|',form_name) AS form_name, LOWER(CONCAT_WS('\n',form_name,parent_route)) AS form_name_s,
CAST(form_id AS CHAR(10)) AS form_id_string FROM Form"
transformer="RegexTransformer"
cacheImpl="SortedMapBackedCache" cacheLookup="strength.form_id" cacheKey="form_id_string">
<field column="form_name" name="form_name" />
<field column="form_name_s" splitBy="\n" />
</entity>
</entity>
</entity>