Keypad Input simulation - 8th digit glitch

Hi boffins. It's me again hoping for some assistance.

I have a project that simulates a user interface whereby the user is asked to enter numerical data a few times. I have discovered it is... hard to do - or else my brain has turned to mush in the 2 weeks since I have had COVID, but I cannot get any methodology to work for me.

I did a search of the boards here and found Phil Mayor's delicious Sweet Shop module and it is EXACTLY what I am hoping to achieve.

After TONS of messing around with variables, I established a very clean set of triggers which simply multiply whatever the previous value is by 10 and then simply add the new keypad input value to that total.

Here is my demo: 

This works fine for the first 7 digits of the sequence, but it just goes weirdly random (or at least I cannot discern the pattern) on the 8th key press. It simply starts adding zeros no matter what key is being pressed.

Is there a limit to the value Storyline 'calculates' values?

Is there an easy way of simply getting a user to type in a numeric string, such as an ID number, or telephone number without all this... ugh... mathematics... <shiver>?

Can anyone discern where I have gone wrong? Or provide an alternative solution that doesn't melt my poor COVID-addled brain?