Facet regex problem with TermsFacetBuilder.size

Elastic Stack Elasticsearch
1

Code:
TermsFacetBuilder facetBuilder = new
TermsFacetBuilder("prefix").field("prefix");
facetBuilder.size(10000);
facetBuilder.facetFilter(boolFilter);
facetBuilder.regex("^[a-z]",Pattern.DOTALL);

the result still contains prefix in [a-z], but if I change size from
10000 to 1000, it works.

very weird~