I've heard back. There's a tolerance option that I can't share (I'll see if I can make it public so I can) for UDF motion definitions; however he's not convinced that's a problem here. Also a TUI option to change the way Fluent works out the solve/donor distance, I can share that.
/define/overset-interfaces/options dpm
Available methods:  [0]: cell volume based   [1]: boundary distance based
That may help. Note, use the 1:1 mesh as the method tends to be more reliable when the cell size matches. It's marginally less critical for stationary meshes as they're easier to check, for moving mesh you'd need to confirm the update was good every few time steps.Â
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