I don't think this has anything to do with your custom tickStrings etc, it happens with stock pyqtgraph too. I've never dug far enough to find out why, but it's like if you enable autorange, but that doesn't require a redraw, then when data is added to the plot (which does appear in correct location and scale etc) then axes labels don't get updated to match. I work around this in a few different ways depending on how my plots are initialised/used/reset, but in general I think it's either by explicitly calling a manual
autoRange after enableAutoRange or by calling
setRange if I know what the initial axes ranges should be.