Hi Bernhard,
Please double-check that you’re using the latest ForestPack release - we’ve made adjustments to this feature in recent versions, and in the latest release the Surface Area > Surf. Mat. ID option should work fine in the UV Surface mode.
Thanks in advance for any further comments on this issue you can share about it with us / our user community (in case some user(s) will run into similar troubles).
Best regards,