I noticed recently when using Player#openSign, the player can no longer edit.
There is kind of 2 issues here.

With the included jar file, I will explain the two issues
setup:

• place a sign down in the world
• while looking at the sign, run command "/test set" (this just caches the block for later)

Issue #1 - can't edit:

• run command "/test open" (this will open the sign)
• type stuff in editor
• close sign editor
• At this point you shall see no changes to the sign, and in console you shall see:
  "Player <YourName> just tried to change non-editable sign"

test code (which is in the provided plugin):

@Override
public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
    if (sender instanceof Player player && args.length == 1) {
        if (args[0].equalsIgnoreCase("set")) {
            block = player.getTargetBlockExact(50);
        } else if (args[0].equalsIgnoreCase("open")) {
            if (block != null && block.getState() instanceof Sign sign) {
                player.openSign(sign);
            }
        }
    }
    return true;
}