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In general, residence permits expire if people leave Germany for more than 6 months and if there is no exceptional agreement with the "Ausländerbehörde". Details on that regulation can be found here, for example.
However, section 51 (7) of the German residence law states the following:
" If a person entitled to asylum or a foreigner whom the Federal Office for Migration and Refugees has incontestably granted refugee status leaves the federal territory, the residence title shall not expire as long as he possesses a valid travel document for refugees issued by a German authority. The foreigner shall have no entitlement to the renewed issuance of a residence title on the basis of his recognition as a person entitled to asylum or by virtue of having been incontestably granted refugee status by the Federal Office for Migration and Refugees if he has left the federal territory and the competence for issuing a travel document has passed to another state."
If I understand correctly, the residence permit of people with a refugee recognition (and a corresponding "blue passport") does not expire until the blue passport is valid. If it expires, the affected person may not have the right/chance to get an extension for the residence permit in Germany anymore. According to section 11 of the Geneva Refugee Convention, the responsibility of the travel document/blue passport issuing will be with the country, in which the person has his/her habitual residence/has settled.
But to be sure, I will link two of our wonderful experts here also. Am I right on this and is there anything to add, @mbeon-Christine or @Mbeon-Lena?
All the best,