Using KMyMoney 5.1.2.
I apologise for the length of this but it is important to show the sequence of events.

I own shares in a US company which pays dividends in USD. My base currency is GBP.
There is an account in GBP which contains the US shares.

I want to enter the reinvest transaction.
Activity is set to Reinvest Dividend.
Security is set to the US company.
Enter the date and number of shares purchased.

Go to the Fees box and select the category for in my case Share Dealing Fees.
Click the split button and enter the fees in USD.
After clicking on the tick I am prompted for an exchange rate.
Enter the rate applicable at the transaction date and click update price history. Enter.
Go to the Interest field on the transaction form.
Enter the category for dividend income and click the split button
Enter the full amount of the dividend in USD without fees.
After clicking on the tick I am prompted for an exchange rate.
Enter the rate applicable at the transaction date and click update price history. Enter.
In the Transaction amount enter the amount reinvested (Dividend minus fees) in USD.
I can now click on Enter and I once again get asked for the exchange rate which I enter.
In the ledger, KMyMoney shows the correct number of shares, the correct price paid, the value is incorrect. On the transaction form the fee amount is shown as dollars but the amount is actually the GBP value.
Errors in red above. Also, why do I get prompted for the exchange rate three times.

Am I doing something wrong or is this possible bugs.

Thank you.

If your shares are really held in an account in GBP, you do indeed need to give an exchange rate for the dividend and for the fees. The fees should be paid from the account, and you should have the exchange rate used for them. Likewise for the dividends. If your shares are not actually held in a GBP account, you need to change the currency of the investment account to USD.

Thanks for the reply. I have now set up an investment account in USD and my USD shares are in that account.
This is what happens with some real numbers.

My share statement says I received a dividend of $500.
Dealing fees were $6.
It says I reinvested 1.934902 shares at $255.31 which multiplied = $493.99982962 which is just about $494

Now I enter this into KMyMoney.
I go into the ledger for the account and select New -> Reinvest Dividend.
I select the security which is the US company.
Enter the date of the transaction, Number of Shares (1.934902) and Price ($255.31).
In the Fees field select the category. In my case an expense category Investments:Share Dealing Fees. Select the split and enter an amount of 6.
Clicking on the tick I am asked for a USD to GBP exchange rate. I enter 0.8316. Update Price History is ticked. Click OK.
Click OK again to exit the split editor.
In the Interest field select the category. In my case an income category Investments:Dividend Payment. Select the split and enter an amount of 500.
Clicking on the tick I am asked for a USD to GBP exchange rate. I enter 0.8316 (in fact it is already set to that value). Update Price History is not ticked this time.
Click OK again to exit the split editor.
Everything is now entered so click Enter to put the transaction in the ledger.
I am asked for a USD to GBP exchange rate again and enter 0.8316.
Update Price History is not ticked this time.
In the ledger entry I see:
Quantity = 1.934902. Correct.
Price = $255.31. Correct.
Value = $498.98 Shouldn't this be either $494 or $500?
In the transaction Form I see:
Shares = 1.934902. Correct.
Price/Share = 255.31. (No $ symbol).
Fee Amount = $4.99. Not correct. This is $6 converted to GBP rounded to 2 decimals.
Interest = $498.98. Again, shouldn't this be either $500 or $494?

Again, what I see as errors highlighted in red.

I don't understand why you still get exchange rate requests if everything is in USD. Is it possible the stock is somehow dependent on a GBP entity? It should not. Make sure the trading currency of the stock is USD. As to the rounding, I always define stock to use "Total for all shares" under "Price entry." That basically says that KMM should never round money paid or received in a transaction but rather round the number of shares. The reason is that money paid or received always has exactly two significant digits; the bank always rounds the number of shares; not the amount.

OK. I think I have it.
The problem was that the categories I was using for share dealing fees and dividends had a currency of GBP. I set up two new categories using USD and when I use those I do not get prompted for exchange rates. Everything seems to be working fine now.
Just one more question. You say you set stocks to use Total For All Shares which I understand but what do you set Remainder to? Round, Ceil, Floor or Truncate? Mine is set to Round at the moment. Any reason why it should be one of the others?
Thank you for all the help.

I believe the default is Round, which is what I use.

Really appreciate the help. Thanks.