I just found that the same is true of tag summary reports. I thought maybe I could get useful monthly income/expense reports by tagging everything, but the IRA distributions show as zero even on the tag reports. This doesn't make sense, since after all the purpose of IRA accounts is to provide income during retirement.
I figured out a work-around for this, which I'll describe in case others have the same problem. I created an asset acoount named IRA Distributions and changed the category of the transactions in the IRA account to transfer to it rather than directly to my checking account. Then I entered transfer transactions from the asset account to the checking account of the same amounts on the same dates.
This fixed the cash flow report. (The expense totals for each month are wrong because it subtracted the transfers from expenses rather than adding them to income -- this doesn't matter because the balance is correct.) However, I still needed a way to get the IRA distributions into my budget. So I added a category with that name and entered dummy transactions in the asset account. To keep them from affecting my net worth and keep the account off the summary page, I marked it as an inactive account, which has no effect on its use except that it must be temporarily activated while adding new transfers in`into it. Now the budget graphs on the summary page are accurate.