I can reproduce the issue on focal but not on groovy. I installed the groovy version of the ibus binaries on focal, but the issue was still present, so this seems not to be an ibus bug.
I can type ü with a German keyboard layout, so it seems to be about dead keys rather than accented letters as such. Furthermore, I can't input e.g. Chinese characters using IBus at all.
I can reproduce the issue on focal but not on groovy. I installed the groovy version of the ibus binaries on focal, but the issue was still present, so this seems not to be an ibus bug.
I can type ü with a German keyboard layout, so it seems to be about dead keys rather than accented letters as such. Furthermore, I can't input e.g. Chinese characters using IBus at all.