This bug is in any version of ExtJS when using Linux and Firefox earlier than version 57, also when using Firefox ESR.
It appears if in Ext.form.field.Date inenter the value - April 1, 1982. and when the deactivate input field occur, the framework automatically changes the value in the input field to March 1, 1982. I see this problem in Ubuntu Linux in Firefox before version 57 and in Firefox ESR as well as other browsers based on Firefox, in the frequency of Waterfox too.

It is interesting that in Windows this bug does not appear, only in Linux, if who has a Mac, check out somebody. The date automatically changes from April 1 to March 1 if the input to field year 1956, 1959, 1982-1984, maybe even some other.

By the way, in ExtJS 6.5.3 I created a screenshot, pay attention - in March, two 31 days: