ACF is not setting the character limit, that is the WP Database Schema. The options_name field is varchar(64).
The WP team has been talking about changing this to varchar(255) for 4 years with no action taken, they keep saying, “maybe in the next release”.
I routinely change this by altering the database and change this column to varchar(255).
However, when you update WP it will be changed back to 64 and break your site unless you do something to prevent that from happening. Which is why I built this https://github.com/Hube2/wp-update-prevent-db-changes
Even at 255, you still need to be mindful of how long the option name will get, especially with nested repeaters.
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