Stunner - unable to set cycle timer with cron expression

Customer needs a timer to execute on the first of every month.

When trying to set 'Timer Settings' property by selecting Fire multiple times,
inputting cron expression like '0 0 0 1 * ?' shows the following validation error.

The time cycle must be a valid cron interval or an expression like #{expression}.

This cron expression can be set by legacy process designer and it works as expected.

Furthermore, when selecting ISO instead of Cron and inputting ISO-8601 expression like 'R/01T/P1M' , it still shows the following validation error.

The timer cycle must be a valid ISO-8601 repetable interval or an expression like #{expression}.