Up to the ex-date a share trades cum-dividend — its price includes the right to the next payment. On the ex-date that right stays with whoever held the share the night before, so a new buyer is paying for a share without it. The price typically opens lower by roughly the dividend: the ex-dividend adjustment.
So a price drop on an ex-date is mostly your own dividend moving out of the share price and into the money owed to you. In the app, the holding's value falls and, usually within a day, the dividend is booked in your Dividend Income — your total is roughly where it was. Roughly, because the market is still moving that morning for every other reason too.
You're not 20p poorer. You're 20p of share price lighter and 20p of dividend heavier.
Educational content only. It is not personal financial advice.