I have recently run across this several times. In most cases, it seems I have multiple securities defined (within the same Investment Account) in some cases because an imported transaction created one of them, in others because the stock had been renamed in the past. In this case, I have Embraer S A Sponsored Adr (ERJ) ans also EMBRAER S A SPONSRD ADR (EMBJ) (note in this case different symbols, and thus different underlying equities. I have a transaction (BUY) using the first one. If I edit the transaction and select the second one, as soon as I leave the Security field (whether by Enter or Tab or just clicking a different field or the "Enter" button, the name reverts, prior to saving if I hit Enter. In addition, I can change the security to a completely different one, but if I edit again and select the second of the above names, it changes to the first as soon as I leave the field. If necessary, I can try capturing a video of it happening. In further addition, if I create a new transaction and enter the first name, it also changes to the second one as soon as I leave the Security field.
I have a similar issue using Google. Google has 2 classes of stock with different symbols. Transactions, like recording a dividend, to one get changed to the other just before saving the transaction.