Adding a Constant Line Pulls Historic Data But It's Blank

Hi folks,

Hope you can help. 

I'm new(ish) to Power Bi, being self taught via videos, reading and good old fashioned trial and error. 

I've created a line and stacked chart, with a constant line for the SLA target (10 days).

This is linked to a date slicer to adjust accordingly. 

Prior to adding the constant line it worked perfectly, however, now I've added the line it adds the previous years month but with blank data and I've not changed the slicer settings (there is data available for 2023). Once I remove the constant line, it reverts back to normal and no issues. Can someone please assist? I've included screen prints of the graph in original state and how it switches once you add the constant line. Thanking youin advance 

Without constant line the data works. 

When constant line is added, it randomly adds last years month with blank data.