Just as @Liza_Dayoub recommended, I would suggest updating the
ignore_above configuration for strings in your index templates associated with your Logstash indices. This can either be done by editing the existing default logstash mapping, or by applying a new template that takes precedence  over the existing ones.
Without knowing exactly how your Logstash mappings are determined, I will try and share a snippet of a new template that would define the behavior I believe you are hoping to modify.
You can find the default template for Logstash and ES 7.x here  You will notice that there is no explicit mention of the
ignore_above: 256 there. That is because this is a global Elasticsearch default on all dynamic fields that are recognized as strings. Here is a blog post that explains this. For more information about Logstash template management you can check out the docs
Does that help?