Bastian,I've been trying to reproduce the issue you are describing but so far everything I've tried has come accross without issue. Creating and outlining a new table in the DDT with two decimal fields (one 18,4 and one 18,5) comes accross just fine after deployment.
Are you on the latest version of the framework? The 1.6.6 beta listed here is actually very stable (it will be released officially as soon as the documentation updates are completed.)