It looks like the CSS class changed from 5.1.1 to 5.1.3 for the datefield icon. Now if you create a pickerfield and try and change the triggerCls to the datefield icon, it won't work because the CSS is too specific.

5.1.1:
Code:
/* */
/* */
/* line 1, ../../../../../../../data/qlp/crm/libraries/ExtJS/extjs-5.1.1.451/packages/ext-theme-neutral/sass/src/form/field/Date.scss */
.x-form-date-trigger {
  background-image: url(images/form/date-trigger.gif); }
5.1.3:
Code:
/* */
/* */
/* line 2, ../../../../../../../data/qlp/crm/libraries/ExtJS/extjs-5.1.3.228/packages/ext-theme-neutral/sass/src/form/field/Date.scss */
.x-form-field-date .x-form-date-trigger {
  background-image: url(images/form/date-trigger.gif); }
Fiddle https://fiddle.sencha.com/#fiddle/1dr4