Problem solved. Almost.
Maybe (!) sometimes it's better to have a look at the log. There, I found dropped packets, drop reason "Virtual defragmentation timeout" ("UDP/0 Traffic Dropped from x.x.x.x to y.y.y.y").
Interesting in that case: only a very specific type of traffic is triggering this issue: RADIUS packets with MF Bit set.
To me, it seems like the R77.30 gateway simply forwards these packets, even if no more fragments arrive. I can see these packets on both incoming and outgoing interface.
The R80.20 gateway drops the first fragment if no other fragment is following, which should be normal behaviour according to sk97666.