This is not a surprise, and given your position as a witness, it is surprising that you expected any other outcome.

The on chain conversion uses the on chain price feed of steem (which as a witness, you and other top 20 witnesses publish !) to calculate how much steem you will effectively receive.

Since the price on chain is now higher than 0.26, you will receive less steem as you are effectively 'buying' at a price higher than which you sold for sbd.

Why is the on chain price higher?

A couple reasons --

HTX is the only venue that trades SBD now, but the wallets are offline. Price discovery cannot be exercised if the vasts majority of SBD that is now not trading on Upbit, cannot migrate to this new venue.

At the same time, the market cap of Steem is at time of writing, about 84 million and per 10% debt limit and haircut rules, the value of SBD will be valued and convert for less than 1 dollar.

The only way things will resolve now, is if steem's mcap increases to about 140 million, or the number of sbd reduces to under 8.4 million.

Both can go in that direction at the same time.