Your symptom sounds like a software bug: after changing the amount, the category is being shifted to the next entry in your category list, so the first save is not preserving the original category. The fact that editing it a second time restores the correct category suggests the category field is being misapplied during the first save rather than you doing anything wrong.
What to do now
- Edit the transaction amount first, then immediately verify the category before clicking OK.
- If the category changes after saving, change it back in the category field and save again, since that appears to stick correctly.
- For a workaround, try changing only the category by double-clicking the category field instead of reopening the full edit window when possible.
- If the transaction is split, use the split transaction editor rather than the single-transaction edit path, since MoneyLine treats split categories differently.
Reporting it
This looks like something MoneyLine support should fix in the editor logic, because a correct save should not advance to the next category automatically. When you contact support, include the exact steps: open transaction, change amount, click OK, category shifts to next item, then edit again and it saves correctly on the second try