Original post is from June, has the problem been fixed yet?
If the turn signal bulbs are not from different manufacturers, then maybe there is one or more faulty ground path?
I thought I had a failed right front turn signal harness on my 78 W116 300SD that turned out to be a poor ground connection.
W116 right front turn signal electrical gremlin
Follow the negative wires (usually brown) from each turn signal socket to where they ground to the chassis.
Disconnect the negative cable from the battery.
Re-establish a good ground by removing the grounding screw, exposing bare metal at the chassis and cleaning
all the ground wires at the connection with electrical contact cleaner before you reconnect them back to the chassis.
Reconnect the negative cable and hopefully enjoy properly working turn signals.