Does anyone know why a field on a custom record form definition would move to a totally different tab after deployment. Both tabs exist in the account (and the form) but the SDF seems to move it with no errors or reason

Not sure, but, does it depend on the script id value in the XML? Wondering if, whilst the tabs exist, the underlying script id does not match so SDF just dumps it. Might be worth a look in the absence of any other responses.

It appears that regardless of what tab you specify for a form in a custom record it ignores and reverts back to the tabs you have defined on the record definition itself. Seems like quite a limitation

Are the custom record template layouts ID's the same in both systems?