Partially reachabilty issues with VS in F5OS tenant

As preparation of our service migration from iSeries to rSeries, we created a test-VS without a pool, but with an iRule responding with a simple static html-website.
This works fine so far, but we must notice, that sometimes this VS is not reachable in the F5OS tenant.
For now we could at least identify that the issue seems to be on the border between F5OS and tenant, because we see incoming packets with tcpdump on F5OS level, but they will not be forwarded into the tenant, because with the tcpdump there it's missing.
Is this behavior related to the VS type of just using an iRule or is there something wrong/corrupt in our configuration?
How can I further verify/troubleshot this, once the issue occurs again? Or which settings should I double check?
We want to be sure to have the correct basic setup available, before migrating the first productive VS to the new platform.

For your reference, we are using route domains in combination with partitions within the tenant.
So we created the VLANs on F5OS level, then deleted them in the Common partition in the tenant and then re-created them again with the same name, but in the correct partition/route domain. And finally created the selfips within the partition/route domain within the tenant.
F5OS: 1.8.3EHF1, TMOS: 17.5.1.3

Thank you!

Regards,
Stefan