This wiring diagram shows -
Indicators - 13,16,45 and 46
Flasher relay (I believe) - 52
Indicator switch (I believe) - 43
Dashboard flasher repeater - 1 and 11
The flasher is common to both directions, so it won’t be that causing the issue.
As suggested in the comments, you could swap the indicators. If possible, I would start by swapping the connectors which can be seen on the wiring diagram a little further back from the indicators, if they are accessible.
The most likely issue is a bad connection that is getting worse as it heats up. Maybe cleaning the connections at the bulbs is all that is needed.
The issue could also be with the flasher bulb connection on the dashboard, since this is also in parallel with the flasher lamps.