Hi Jason,
I ran into the same issue earlier this year. I opened a support case, and the successful workflow for me was:
1. Publish by right-clicking on the mosaic dataset in Pro > Sharing > Share As Web Layer, ensuring your active portal is ArcGIS Online
2. Choose the 'Cache locally' option to save credits, but in the end everything will be automatically uploaded and prepared in ArcGIS Online as part of the publishing/caching process
That's it! Resampling worked for me after that - hope the same for you.
Chris