DNS-transfer issue when using api to change records

Hi ISPConfig friends!

Perhaps someone can help me with this:

I have 2 ISPConfig servers, one hosts DNS-Zones (lets call him dns1) and one hosts DNS-Slave-Zones (dns2).
Everything works fine as long as a use the ISPConfig webinterface to change records.
Example:

I create a TXT Record test.mydomain.de. = "test-record-state1" in a DNS-Zone on DNS1
-> The record gets transferred to DNS2 correctly
dig @dns1 -t TXT test.mydomain.de returns "test-record-state-1"
some minutes later
dig @dns2 -t TXT test.mydomain.de returns "test-record-state-1"

If i change test.mydomain.de. to be "test-record-state2" via ISPConfigwebinterface on DNS1
-> The change gets transferred to DNS2 correctly

_BUT_:

If i use a "remote user" to change the value of test.mydomain.de. on DNS1 via API, then the change does not get transferred to DNS2:

dig @dns1 -t TXT test.mydomain.de returns "new-value"
dig @dns2 -t TXT test.mydomain.de returns "old-value"

running /usr/local/ispconfig/server/server.sh on both servers does not help.
If i delete the slave zone and recreate it, the new value gets transferred.
Anyone an idee about that?

I use this to renew letsencrypt certificates on a Servers which are is not reachable via http.
Yours from Giessen, Germany
Gerd