This is the result of a known issue. FTP control-channel traffic on port 21 fails to pass through a SNAT. When an FTP request is initiated to a SNAT object, the BIG-IP system does not send any packets out the server-side VLAN, and immediately closes the client-side connection with a TCP reset.
F5 Networks Product Development tracked this issue as CR94277 and it was fixed in BIG-IP LTM version 9.4.5. For information about upgrading, refer to the BIG-IP LTM Release Notes.
Additionally, this issue was fixed in Hotfix-BIG-IP-9.4.4-73.1-HF1 issued for BIG-IP version 9.4.4 for the 9.4 software branch. You may download this hotfix or later versions of the hotfix from the F5 Networks Downloads site.
For information about the F5 Networks hotfix policy, refer to SOL4918: Overview of F5 Networks critical issue hotfix policy.
For information about managing F5 Networks product hotfixes, refer to SOL6845: Managing F5 Networks product hotfixes.