Write to tag fails until project has been saved in designer

I had a problem where a simple button was reporting as not function when it came to doing its job in the PLC.

When I came to test the button in designer (the button has a tag write on both mouse pressed / mouse released) at some point the button failed to function in designer (after I had toggled the PLC bit manually from RSLogix 500).

I then entered an extra rung on the button - “print 1”
This failed to print to the console
Having experienced this before I clicked on Save project and then both the tag write and the print function both started to work.

I cannot say for sure how this compares with the client experiencing a sudden malfunction since I am within designer doing the clicking but I have seen this happen many times before.

I have Ignition 7.6.2

Is the button supposed to write to a PLC tag? Is it writing from the Designer but not form the Client?
Do you see anything in the Client console after the button is pressed? (Top menu > Help > Diagnostics > Console)