cannot create additional X-Axis Constant line

I cannot create additional X-axis constant lines after I added some constant lines already.

Here's the steps to reproduce the problem:

- open a new report; enter or import some data, e.g.

sample data

- create a line chart with dates on X-axis

- duplicate the page with the line chart

- return to the first page ("Page 1") and add an X-axis constant line to the chart

line 1

 - go to the second page ("Duplicate of Page 1") and add an X-axis constant line there as well

line2

 - return to the first page and add the 2nd X-axis constant line to the chart

line3

 - finally, go the second page and try to add the 2nd X-axis constant line there - I always fail, I don't see any option to add it

line4

(There's a workaround - save, close, and re-open your report - X-Axis constant line option will re-appear, but untill you add a few more constant lines.)

UPDATE: I've just found another workaround: instead of closing the report, return to the first page and rename(!) the first(!) X-Axis constant line:

renamed line 1

 Now you can go to the second page and create more X-Axis constant lines:

now line 4 can be created

Could you please fix it?