Thank you for reporting the issue. Can you please try to check if this has already been fixed by using the most recent AppImage version of the application? You can find instructions on how to run it and a link to the software on https://kmymoney.org/appimage. Please report back, if the problem is still present or not. Thanks in advance.

Problem still there with kmymoney-5.1-22-linux-centos_64-gcc.AppImage

In the following, I ignore Date and Memo fields; they're always there.

Working out what "should" happen has been a learning experience for me.  It seems to be, in part, a UK vs US difference, explained below.

Actual behaviour:

I set up a new KmyMoney database, added Investment accounts from the setup wizard,  and an equity account under Investments.  To that I added one equity.  New->Buy shares.  The panel that appears has entry fields for Activity-Buy Shares, Security, Account, Fees category and amount, Interest category and amount, and Memo, Date, Shares, Price/share.  I entered a number of shares, a price, a fee amount (account Commissions), and some Interest (category Dividend Interest).  It accepted all the input, but ignored the Interest amount.

Trying Reinvest Dividend, the field for Account disappears, as do the amount fields for Fees and Interest, even if I set their categories to something sensible.  I can enter some numbers using the split transaction facility.  The Fees amount gets included in the transaction, but the Interest is set to Shares*Price + Fees, irrespective of anything I entered as a split transaction.  That appeared in the Dividend Income category, where I'd sent it, so at least it's consistent.  You can't enter the transaction without specifying an Interest category.  Even if you specify a Fees category, there's no box to enter the number; you have to use the split transaction facility.

Expected behaviour:
Buy Shares should have no Interest category or number, and simply take money from whichever account was specified, that money being Shares*Price + Fees.

For Reinvest dividend, having dug around on the Internet, I think it may be the US vs UK (and presumably elsewhere) difference.  US companies seem to be happy to issue fractional shares, so they appear to offer Share Reinvestment programs that work as the existing kmymoney method - the "number" of shares is (Dividend/Price) and can be fractional.  I think that could happen with UK Unit Trusts (they have a new name these days; whatever), but not with normal Equities, and I was trying to handle the Dividend Re-Investment Plan (DRIP) of a normal UK equity.

UK Equity DRIPs work by having a cash account for each participating shareholder, integer share purchases, and the residual between the Dividend paid and the (Shares*Price) cost going to or coming from that cash account.

I guess if kmymoney were to handle this UK usage, what should happen would be:
There should be Security, Fees category and amount, Interest category and amount, Residual category (no amount), Shares and Price.
The Interest would go to the Interest category;
(Interest - (Shares*Price + Fees)) would go to Residuals category
(Shares*Price+Fees) would be handled as a normal Buy Shares;
effectively, this is a split transaction funded by Interest and Residual.  The split is to ensure the database records the correct Dividend interest received, so the Investment performance report is correct.

Working that out (and it's taken me a couple of hours) explains why I couldn't understand one of my broker's reports!  They effectively have brokerage account per security, and when the amount therein is sufficient, they buy shares from that account, which, AFAIR they call "Cash".  I handle that broker's report items as two transactions, a Dividend and a separate Buy Shares.
Conversely, a few of my direct holdings provide their own DRIP, and they are the ones I'd handle as written above.

I guess if that's not easy to implement, I can just handle these DRIPs as two transactions.  But the kmymoney handbook needs expanding in either case to explain more about how it handles Divident reinvestment activities.

Hope that all makes sense....

Just updating status.