Multi-category axis subplots vertical line

Hello, first time posting.

I am making a figure with two subplots with shared xaxes and a two-level multi-category axis:

(I have anonymized the data somewhat).
The lower plot looks fine but in the upper plot I have these unwatned vertical lines stemming from the categories. Is this a bug and how can I get rid of these?

Furthermore, say I wanted to remove the tick labels on the x-axis of the lower level (the ticks 6XXX-123), is this possible while still showing the upper level (the ticks: 1, 2, 3…)?