@BenClark - There might be a defect here that's worth investigating on our end, but in the meanwhile I have a couple of thoughts/things you could try as a workaround.
1. Before using the event joiner, consider shortening the field names in your source definitions so that the resulting joined definition ultimately has shorter fields too. You might even want to go so far as to shorten the definition name feeding into the event joiner since these names are also prepended in the event joiners definition field names. This entire suggestion would potentially mean using a field mapper before the event joiner since the geotab fields associated with your input definitions are fixed. This means geotab input (i.e. the fixed definition) -> field mapper (where you've defined shorter field names and a shorter definition name instead of using the fixed geotab definition) -> event joiner -> *optional* field mapper (to redefine/shorten the joined field names once more) -> output feature layer.
2. Should the above be a no go, you might want to try truncating the resulting fields even further by removing "geotab_exceptionevent___" as a prefix with your current configuration. This means geotab input -> event joiner -> field mapper (where you reduce the field names even further) -> output. This might alleviate the drop down problem you mentioned before if I recall correctly.
3. The event joiner strips away any tags (such as the TRACK_ID tag). This goes hand in hand with all the suggestions here but make sure you assign the TRACK_ID tag again in the target definition you're specifying in the field mapper processor (after the event joiner). I assume you've done this already, but its worth double checking.