Consider a Java bean with a potentially multi-line String field myField and associated config.jelly file

    <f:entry title="${%My field}" field="myField">
      <f:expandableTextbox />
    </f:entry>

If myField has a multi-line value (as decided by calling hudson.Functions.isMultiline(String)) when the web form is first loaded then everything works as expected - the Descriptor's doCheck methods get called as expected (i.e. as documented).
However, if the field value is either empty or is just a single-line value (i.e. when expandableTextbox.jelly's test="${h.isMultiline(value)}" is not true) then <f:expandableTextbox /> will never call the doCheck methods. That's a bug - it's not working as documented (or as expected).
If a doCheck method is present, it should get called, regardless whether a field value is being rendered as a textarea, textbox or expandableTextbox.

Impact:
This means that users entering data for the first time will not have their data validated.
It also means that plugin developers can loose a few hours trying to figure out why one or more of their carefully-coded doCheck methods are being silently ignored most of the time...

Workaround:
Developers could use <f:textarea /> ... but that leads to a cluttered UI as it always uses a minimum height of 5 rows (as set by ${h.determineRows(value)} which calls hudson.Functions.determineRows(String) which returns a minimum of 5 even for an empty string).

TL;DR: expandableTextbox should not be an exception to the general rule regarding form validation.