I am having a similar problem.
Here is my setup:
Platform: Generic
Version: TrueNAS-12.0-U5.1
Intel(R) Xeon(R) CPU E3-1231 v3 @ 3.40GHz
dirfiler assigned as owner and group of dataset: pcphotos in pool: windowspool
Sharing as windows samba share named photos and it is enabled
when I originally created the dataset the owner was root and group was wheel
When I assign users to wheel I can log in to share without problem and I can read, write, append and delete without problem.
I run into permission issues when I change the owner and group of the dataset. Please note I have made changes without stripping and also with stripping ACLs. I also tried with applying permissions recursively and not as well. I too have checked Apply permissions to child datasets. I think I have tried every permutation, I could be wrong about that, but I always get denied.
It will only work when I revert group owner to wheel and I assign users to wheel group, whether as primary or auxiliary.
Also, I have also tried adding an acl item with root as owner, wheel as group and extra group dirfiler, all with full control permissions and it still did not work.
I have tried following the manual to no success and I also tried following this tutorial
https://www.youtube.com/watch?v=k0X0geU6NOA
Please let me know if you need any other details that may allow you to provide some guidance.
Thank you for your time.