QGIS » QGIS Application

QGIS 2.18.15 on Windows 7. I have a layer linked to a File Geodatabase through ESRI's API, this is joined to two tables in a Personal Geodatabase, one that gives a record a code, the other that turns the code into something meaningful for humans to read. I am trying to symbolize the layer based off the human-readable code field.

In the layer properties symbology tab I see all my categories and associated values, in the attribute table i can see the records that align with those values. When i first open the project the layer displays all symbology as NULL/empty value in the categorization. If I then open the layer properties I see all the categorization values and if I just hit "Apply" the map redraws with proper categorization. However if I zoom in/out slowly the layer draws properly, but if I zoom quickly, the symbology redraws as if all the values were NULL/empty, if I then scroll back to a scale that was drawing properly it sometimes redraws the proper symbol values but if I re-zoon quickly it seems to hold onto the NULL/empty value symbology.

Pure speculation here: it seems like the values from the joined table are not being read fast enough when the screen redraws and so the values are set at nothing when it categorizes the records, thus the symbology displays the NULL/empty symbol in the categorization. There are about 5000 records to read and draw each time a zoom happens. For the screen redraws it seems that some zoom scales remain in a cache and therefore revert to the proper symbology when zoomed to, but once the "NULL/empty symbology is applied in a fast zoom and that replaces the previous version in the cache, only the NULL/empty symbology version shows at that scale.