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I am not sure. I have always done it via robiniaswap just like you did here... Just make sure you enter the total amount to delegate, not just the amount you want to add.

😀I just tried steemword and edit. I check metamask stake and 0.

Did it undelegate it?

No. First time delegated 25 steem im robiniaswap. Second time I edit in steemword and delegate 30 steem to robiniaswap. In Robiniaswap now I see 0 staked?🤔 I testing.

On steemworld it shows that you have 30 sp delegated to robiniaswap...