when exporting photos with custom settings, the sequence number and export name reset every time. when using a custom setting that isn't one of the presets, the number resets as well.

That’s not a bug, but as designed. Lightroom does not know which sequence it should continue, because you could be using more than one sequence. Check the last number you used, then fill in the next number as the Start Number in this dialog if you want to continue that particular sequence.