Large Objects in serializable transaction question

I have a testprogram (using libpq) reading data from a cursor and large objects according to the result of the cursor.
Thecursor is opened in a serializable transaction. 

Just for test reasons I know tried the following:
I started the test program that reads the data from the cursor and that reads the large objects according to the result
ofthe fetch. While the test was running I now was dropping all large objects in a parallel session. Since I am using a
serializabletransaction in the test program I still should be able to read all the large objects, even if I drop them
ina parallel session. But it does not work. I get an error, that the large object can't be opened. 

I now tried the same with with two simple psql sessions. Here it works the way I expected it to.

Has anybody an explanation for that? Is it due to the libpq functions lo_open ...?

Thanks
Andreas
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