If the rears are LED then you would need either an electronic flasher or load resistors to control the flash rate - otherwise BOTH sides should be on the fast-flash rate. Since you have fast flash on at least one side, it's safe to assume that you don't have electronic flasher relay installed.
The reason that they flash fast is specifically to tell you that there is a bulb out - the relay operates in this mode when it has less current through it. The relay 'thinks' that a LED is a 'burned' bulb, because they draw so much less current than a conventional incandescent bulb.
So LED being 'out' or disconnected would not cause your problem per se (as far as relay is concerned LED connected or not is same thing i.e. insufficient current to operate in normal mode);
but if you had load resistors connected on one side only, this would cause the problem you are seeing.
If it was normal at one time then perhaps that connection to the load resistor on the right side LED is broken. If it has always been like that, then it was wired incorrectly to begin with.
Your best ultimate solution is to ditch load resistors anyway & install a plug n play Electronic Flasher Module.
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