@dave43
For some reason this strange behavior has happened to a few other Desktop users.
Their solution was to switch to Forms Mode, then open and delete the "Social Security Benefits Worksheet".
Then returning to the interview, and their SS entry proceeded as expected in the SSA-1099 entry area.
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Or...maybe a computer reboot might help if still whacky. (wild guess)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*