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For several versions now, there has been a Data Merge bug in InDesign that has the potential to trash the template if you use preview on a Multiple Records layout. As far as I know, this has not yet been fixed.
Try rebuilding your template, and outputting it without previewing.
Thank you for the info, SJRiegel. I'll avoid the Multiple Records layout until fixed. Good to know I'm not going senile, either!
Multiple record merges should work fine, as long as you don't use preview (or undo after pushing the preview button and before doing the actual merge). Most times you can rebuild your template by making a new file the same size, then copying the placeholders from the original and using paste in place.