Thanks for posting here in the Community, @Manyhats.
Allow me to share some insights about this Simple IRA concern.
We currently have an ongoing issue wherein Simple IRA contributions are not calculating correctly when payroll. Our developers are not diligently working to get this fix in a timely manner.
For now, I'd suggest reaching out to our Customer Care Team so you'll be added to the list of the affected users. This way you'll be able to receive email notifications about the updates of the issue.
You can reach them through the contact information through this link: Contact the QuickBooks Online Customer Support team.
Thank you for your patience while this was being worked on. Please know that the Community has your back should you have any questions.