I can only add 18
Let's try to upgrade to 10.5. Should this not resolve the issue, a support case would be the next step.
Just so we can reproduce it more efficiently, would it be possible to get a screenshot?
If I add a 19th it does not save it. The weird thing is, that I have another custom property that has 25. and it will not allow me to add any more either. Could there be a upper limit overall in the properties.
We have 180 total values across all of the properties.
What version are you presently running? We fixed a very similar issue in 10.5.
We have scheduled the upgrade for next Tuesday. i will let you know