@pitrh Just did the switch from isc-dhcp-client to dhcpleased today and it played about 30 minutes with the ISP DHCP server the "init, offer, request, nak, repeat"-game before the server finally acked the IP it issued a few seconds before (it was always the same, though). I remembered, that this was always an issue and i saw it with ISCs dhclient, too. I don't know exactly, which software Deutsche Glasfaser is using, but it looks weird from my end. On the other hand better a shitty dhcp server software than having to use pppoe on fibre, which a lot of ISPs seems to prefer in Germany.
Also thanks to @florian for this software. Looking forward, to maybe ditch dhcpcd in favor for the DHCPv6-PD part.